Chapter XV: Appendix: Work for the Telescope

To make at first hand the acquaintance of the celestial bodies is, in and of itself, worth the while, as leading the mind to a new sense of ultimate values. To tell the truth the modern man on the whole knows the Heavens less intimately than did his ancestors. He glances at his wrist-watch to learn the hour and at the almanac to identify the day. The rising and setting of the constellations, the wandering of the planets among the stars, the seasonal shifting of the sun’s path—all these are a sealed book to him, and the intricate mysteries that lie in the background are quite unsuspected.

The telescope is the lifter of the cosmic veil, and even for merely disclosing the spectacular is a source of far-reaching enlightenment. But for the serious student it offers opportunities for the genuine advancement of human knowledge that are hard to underestimate. It is true that the great modern observatories can gather information on a scale that staggers the private investigator. But in this matter fortune favors the pertinacious, and the observer who settles to a line of deliberate investigation and patiently follows it is likely to find his reward. There is so much within the reach of powerful instruments only, that these are in the main turned to their own particular spheres of usefulness.

For modest equipment there is still plenty of work to do. The study of variable stars offers a vast field for exploration, most fruitful perhaps with respect to the irregular and long-period changes of which our own Sun offers an example. Even in solar study there are transient phenomena of sudden eruptions and of swift changes that escape the eye of the spectro-heliograph, and admirable work can be done, and has been done, with small telescopes in studying the spectra of sun spots

Temporary stars visible to the naked eye or to the smallest instruments turn up every few years and their discovery has usually fallen to the lot of the somewhat rare astronomer, professional or amateur, who knows the field of stars as he knows the alphabet. The last three important novæ fell to the amateurs—two to the same man. Comets are to be had for the seeking by the persistent observer with an instrument of fair light-grasp and field; one distinguished amateur found a pair within a few days, acting on the theory that small comets are really common and should be looked for—most easily by one who knows his nebulæ, it should be added.

And within our small planetary system lies labor sufficient for generations. We know little even about the superficial characters of the planets, still less about their real physical condition. We are not even sure about the rotation periods of Venus and Neptune. The clue to many of the mysteries requires eternal vigilance rather than powerful equipment, for the appearance of temporary changes may tell the whole story. The old generation of astronomers who believed in the complete inviolability of celestial order has been for the most part gathered to its fathers, and we now realize that change is the law of the universe. Within the solar system there are planetary surfaces to be watched, asteroids to be scanned for variability or change of it, meteor swarms to be correlated with their sources, occultations to be minutely examined, and when one runs short of these, our nearest neighbor the Moon offers a wild and physically unknown country for exploration. It is suspected with good reason of dynamic changes, to say nothing of the possible last remnants of organic life.

Much of this work is well within the useful range of instruments of three to six inches aperture. The strategy of successful investigation is in turning attention upon those things which are within the scope of one’s equipment, and selecting those which give promise of yielding to a well directed attack. And to this end efforts correlated with those of others are earnestly to be advised. It is hard to say too much of the usefulness of directed energies like those of the Variable Star Association and similar bodies. They not only organize activities to an important common end, but strengthen the morale of the individual observer.

INDEX

A

Abbé, roof prism, 162

Aberration, compensated by minute change of focus, 266
illuminates the diffraction minima, 265
relation determines of focus and aperture, 266

Achromatic long relief ocular, 146
objective, 77

Achromatism, condition for, 78
determination of, 78
imperfection of, 87

Adjustment where Polaris invisible, 235

Air waves, length of, 255

Alt-azimuth mount for reflector, 102
mounts, with slow motions, 102
setting up an, 228

Anastigmats, 84

Annealing, pattern of strain, 68

Astigmatism, 84, 209
of figure, 210

Astronomy, dawn of popular, 19

B

Bacon, Roger, alleged description of telescopes, 6

Barlow lens, 152

“Bent,” objective, 86

Binocular, 2
advantage of, exaggerated, 151
for strictly astronomical use, 152
telescopes for astronomical use, 163

C

Camouflage, in optical patents, 97

Cassegrain, design for reflecting telescope, 22

Cassegrain, sculptor and founder of statues, 22

Cell, taking off from a telescope, 202

Chromatic aberration, 11, 76
investigation of, 210
correction, differences in, 91
error of the eye, 90

Clairault’s condition, 81
two cemented forms for, 81

Clarks, portable equatorial mounting, 109
terrestrial prismatic eyepiece, 158

Clock, the cosmic, 233

Clock drive, 110, 174

Clock mechanism, regulating rate of motor, 179

Coddington lens, 137

Cœlostat constructions, 126
tower telescopes, 127

Color correction, commonly used, 211
examined by spectroscope, 211
of the great makers, 90

Coma-free, condition combined with Clairault’s, 83

Comet seeker, Caroline Herschel’s 118
seekers with triple objective, 119

Crowns distinguished from flints, 64

Curves, struggle for non-spherical, 18

D

Davon micro-telescope, 148

Dawes’ Limit, 261
in physiological factors, 263

Declination circle, 108
adjustment of, 239

Declination circle, adjustment by, 237
facilitates setting up instrument, 110

Definition condition for excellence of, 254
good in situations widely different, 254

DeRheita, 12
constructed binoculars, 13
terrestrial ocular, 13

Descartes’ dioptrics, publication of, 11
lens with elliptical curvature, 12

Dew cap, 219

Diaphragms, importance of, 43

Diffraction figure for bright line, 269
pattern, 256
solid, apparent diameter of, 262
solid of planet, 269
solid for a star, 260
spectra, 190
system, scale of, 260
varies inversely with aperture, 260
through objective, 258

Digges, account suggests camera obscura, 7

Dimensions, customary, telescope of, 24

Discs, inspection of glass, 66
roughing to form, 69

Distortion, 86

Dolland, John, 28
published his discovery of achromatism, 29
Peter, early triple objective, 29

Dome wholly of galvanized iron, 250

Domes, 246

Driving clock, a simple, 174
pendulum controlled, 177
clocks spring operated, 175

E

English equatorial, 110
mounts, mechanical stability of, 113

Equatorial, adjustments of, 230

Equatorial, coudé, 124
mount, different situations in using, 229
mount, first by Short, 104
mount, pier overhung, 115
mount in section, 107
two motions necessary in, 106

Equilibrating levers, devised by T. Grubb, 39

Evershed, direct vision solar spectroscope, 189

Eye lens, simple, preferred by Sir W. Herschel, 136

Eyepiece, compensating, 142
Huygenian, 139
Huygenian, achromatism of, 140
Huygenian, with cross wires, 140
Huygenian, field of, 141
Huygenian focal length of, 143
measuring focus of, 136
microscope form, 147, 148
monocentric, 139
a simple microscope, 134
Tolles solid, 141

F

Field, curvature of, 85
glass, arrangement of parts, 151
Galilean, 150
lens diameter possible, 150

Field lens, 139

Figuring locally, 73
process of, 73

Filar micrometer, 172

Finder, 108, 132
adjustment of, 230

Fine grinding, 69

Fixed eyepiece mounts, 118

Flints, highly refractive due to Guinand, 36

Foucault, 39
development of silver on glass reflector, 41
knife edge test, 212

Foucault, methods of working and testing, 41

Fraunhofer, 36
applied condition of absence of coma, 82
form of objectives, 37
long list of notable achievements, 38

“Front view” telescope, 32
mechanical difficulty of, 33

Furnaces, glass, classes of, 59

G

Galilean telescope, small field of, 9

Galileo, exhibited telescope to senators of Venice, 8
grasps the general principles, 7
produces instrument magnifying 32 times, 8

Gascoigne, William, first using genuine micrometer, 12

Gauss, Objective, 82

Gerrish, application of drive, 181
motor drive, 179

Ghosts, 137

Glass, dark, as sunshade, 166
forming and annealing, 62
inspection of raw, 61
losses by volatilization, 58
materials of, 59
origin of, 57
persistent bubbles in, 58
a solid solution, 57

Grating spectroscopes, 190

Gratings, spectroscope, 189

Gregory, James, described construction which bears his name, 19
failed of material success, 20

Grubb, Sir Howard, objectives, 74

Guinand, Pierre Louis, improvements in optical glass, 36

H

Hadley, disclosed test for true figure, 27
John, real inventor of reflector, 25

Hadley’s reflector, tested with satisfactory results, 26

Hall, Chester Moor, designed first achromatic telescope, 27
had telescopes made as early as 1733, 27

Hand telescope, magnifying power, 150
monocular, 151

Hartmann test, 213
on large objectives, 267
principle of, 214

Hartness, turret telescope, 130, 131

Heliometer, principle of, 171

Hensoldt, prism form, 163

Herschel’s discovery of Uranus, 32
forty foot telescope, 34
Sir John, 35
Sir John, proposed defining condition, 81
Sir William, 31

Herschel’s time, instruments of, 35

Hevelius, construction for objective of 150 feet, 17
directions for designing Galilean and Keplerian telescopes, 14
invention of first periscope, 15
Johannes, 13
mention of advantage of plano convex lens, 14
mentions telescope due to DeRheita, 14

Housing reflector of 36 inch aperture, 243
rolling on track, 242
simplest instrument for fixed, 241

Huygens, Christian, devised methods of grinding & polishing, 16

Huygens’ eyepiece, introduction of, 24

Huygens, sketch of Mars, 16

I

Image, correct extra focal, 208
critical examination of, 204

Image, curvature of, 87
seen without eyepiece, 134
showing unsymmetrical coloring, 208

Interference rings, eccentric, 205

Irradiation, 262

J

Jansen, Zacharius, 4

K

Kellner, ocular, 145

Kepler, astronomical telescope, 10
differences of from Galilean form, 10

Knife edge test of parabolic mirror, 212

L

Lacquer, endurance of coating, 223

Latitude scale, 232

Lenses, determinate forms for, 80

Lens, magnifying power of, 134
“crossed,” 24
polishing the fine ground, 70
power of, 78
triple cemented, a useful ocular, 138
simple achromatic, 137
single, has small field, 137
spotted, cleaning of, 217

Light grasp and resolving power, 265
small telescope fails in, 264

Light ratio of star magnitudes, 264

Light transmitted by glass, 53

Lippershey, Jan, 2
discovery, when made, 5
retainer to, 3

Lunette à Napoleon Troisiéme, 154, 155, 162

M

Magnifying power, directly as ratio of increase in tangent, 135
powers, increase of, 273

Marius, Simon, 5
used with glasses from spectacles, 5

Marius, picked up satellites of Jupiter, 5

Meridian photometer, 194

Metius, James, 4

Metius, tale of, 4

Micrometer, double image, 171
square bar, 171

Micrometers, 168

Micrometry, foundations of, 12

Mirror’s, aberrations of, 92
adjustment of, 206
concave spherical, 92
final burnishing of, 226
hyperboloidal, 96
lacquer coating for surface, 221
mounting, by Browning, 49
parabolic oblique, shows aberration, 95
surface, prevention of injury to, 220

Mittenzwey ocular, 141

Mountain stations, good or very bad, 254

Mounts, alt-azimuth and equatorial, 98

Myopia, glasses for, came slowly, 2

N

Navicula Lyra, stages of resolution of, 271

Newton, abandoned parabolic mirror, 21
blunder in experiment, 20
gave little information about material for mirrors, 23
Isaac, attempt at a reflector, 20

Normal spectra, 190

O

Objective, adjustable mount for, 44
adjusting screws of, 44
Clark’s form, 83
cleansing, 203
examination of, 202

Objective, four-part, 85
Fraunhofer flint-ahead, 83
how to clean, 216
spacers, to take out, 217
typical striæ in, 203

Objective prism, photographing with, 185, 187

Objectives, crown glass equiconvex, 80
over-achromatized, 90
rated on focal length for green 24

Observatories, cost of Romsey, 252

Observatory at small expense, 249
Romsey, description of, 249
with simple sliding roof, 245

Observing box, 229

Oblique fork alt-azimuth, 100

Ocular, apparent angular field of, 146
terrestrial, 147
Tolles terrestrial, 147
typical form, 45

Oculars, radius of curvature of image in, 146
undesirability of short focus, 275

Open fork mount, 115
well suited to big reflectors, 117

Optical axis, to adjust declination of, 238

Optical glass, classes of, 63
data and analysis of, 64
industry, due to single man, 36
production of, 60

Orthoscopic ocular, 145

P

Parallactic mount, 104

Petition for annulment of Dolland’s patent, 29

Photometer, artificial star Zöllner, 194
extinction, 198
photoelectric cell, 199
precision of astronomical, 199
selenium cell, 199
Zöllner, 197

Photometers, three classes in stellar, 193

“Photo-visual, objective,” 89

Pillar-and-claw stand, 98

Pillar mount, 240

Pitch, optician’s, 71

Placement for tripod legs, 236

Polar and coudé forms of reflector, 125
axis, adjustment of by level, 232
axis, alignment to meridian, 232
axis, setting with finder altitude of, 234
telescope, 119, 122

Polaris, hour angle of, 233
a variable star, 199

Polarizing photometer, 193

Pole, position, 234

Polishing machine, 70
surface of tool, 72
tool, 71

Porro’s second form, 157
work, original description of, 156

Porta, description unintelligible, 7

Portable equatorial, adjustment of, 230
telescopes, mounting of, 228

Porter polar reflector, 130

Position angle micrometer of Lowell Observatory, 173

Powers, lowest practicable, 276

Prismatic inversion, Porro’s first form, 155

Prismatic inverting system, the first, 154

Prisms, Dove’s, 154

Prism field glasses, stereoscopic effect of, 159

Prism glass, 152
loss of light in, 160
objectives of, 161
weak points of, 160

R

Resolving constant, magnification to develop, 275
power and verity of detail, 2
power of the eye, 274

Reticulated micrometer, 169

Reversion prism, 153

Right ascension circle, 108

Ring micrometer, 169
computation of results of, 170

Ring system faults due to strain, 205

“Romsey” observatory type, 248

Rack motion in altitude, 100

Ramsden, ocular, 144

Reflection, coefficient of, from silvered surface, 54

Reflector costs, 55
cover for, 242
development in England, 41
for astrophysical work, 56
light-grasp of, 53
relative aperture of, 50
section of Newtonian, 45
skeleton construction, 49
suffers from scattered light, 56
working field of, 55

Refractive index, 63

Refractors and reflectors, relative advantages of, 52
few made after advent of reflector, 27
in section, 43
light transmission of, 53

Refractors, relative equivalent apertures of, 54
tubes of, 42

S

Scheiner, Christopher, use of Kepler’s telescope, 11
devised parallactic mount, 11

Secondary spectrum, 87
new glasses reducing, 88

Seeing, 257
conditions, for difference of aperture, 257
conditions generally bad, 253
standard scale of, 256
true inwardness of bad, 253

Separating power, to compute, 261

Short, James, mastered art of figuring paraboloid, 27
took up Gregorian construction with success, 27

Shortened telescope, 152

Sights, on portable mount, 229

Silver films, condition of, 54

Silvering, Ludin’s process, 225
processes, 222
process, Dr. Brashear’s, 222

Sine condition, Abbé’s, 82

Slit, spectroscope, Abbé type, 184

Snow cœlostat telescope, 127

Solar diagonal, 166
eye piece diaphragms in, 168
early spectroscopes, 188
polarizing eyepiece, 167
spectroscope, 187

Spacers, 44, 218

Spectacle lenses, combination of, 2

Spectacles for presbyopia, 2
invention of, 1

Spectra, visibility of stellar, 183

Spectro-heliograph, principle of, 191
simple type of Hale’s, 191

Spectroscope, 182
construction of astronomical, 182
of Lowell refractor, 185
ocular, McClean form, 183

Specula, small, methods of support, 49

Speculum metal composition of, 24

Sphenoid prisms, 158, 163

Spherical aberration, 11
amount of, 80
annulling in both directions, 84
examination for, 207
quick test of, 267
remedy for, 79
concave mirror, errors of, 22

Star, appearance of, 204
artificial, 66, 203
diagonal, 165
disc, apparent diameter of, 259
image of reflector, 206

Steinheil, achromatic ocular, 144
Karl August, silvering specula, 39

Striæ, location of, 67

Surface, treatment of deterioration of, 218

T

Taylor, triplets with reduced secondary spectrum, 89

Telescopes, choice and purchase of, 201
Early in 1610 made in England, 6
first, 3
the first astronomical, 9
improvement of early, 11
lineage of, 1
name devised, 9

Telescopes, portable and fixed, 108
1609, for sale in Paris, 5
size and mounting of early, 14

Telescopic vision, discovery of, 2

Templets, designed curves of, 69

Tests for striæ and annealing, 68

Transparency, lack of in atmosphere, 255

Triplet, cemented, 85

Turret housing of reflector, 244

V

Variable stars, 192

W

Wedge calibrated by observation, 197
photographic, 197
photometer, 197

Wind, shelter from, 240

Z

Zeiss, binocular of extreme stereoscopic effect, 161

Zöllner, photometer modification of, 198

Zonal aberration, 209

* * * * *

Transcriber's Notes

Obvious typographical errors have been silently corrected. Variations in hyphenation and accents have been standardised but all other spelling and punctuation remains unchanged.

Italics are represented thus _italic_, bold thus =bold=, subscript thus _{s} and underline thus underline=.

In caption of Fig. 49.—Spherical Aberration of Concave Lens. Concave has been changed to Convex

In “An objective of 4.56′ inches aperture has a resolving constant of 1″ and to develop this should take a magnification of say 300,” 1″ has been hand altered in the original and may be 1′.

The table “Characteristics of Optical Glasses″ has been divided to fit within the width restriction.