SCIENCE. 



565 



the observations, and a possibility of prolonging them 

 over longer intervals. A similar effect is produced by the 

 constant direction of the line of sight. No especial ob- 

 serving chair is needed. There is no limit to the size of 

 the attachments which may be made to the eyepiece, 

 since they need not be moved. This is a great advantage 

 in certain spectroscopic and photometric measurements. 

 A strong wind interferes seriously with many observa- 

 tions, as it is impossible to make a telescope so stiff that 

 it will not be shaken by sudden gusts. In the plan here 

 proposed the mirror alone is exposed, and its surface is 

 too small to give trouble. 



By means of a* long handle the position of the mirror 

 may be regulated from the eye-end, and the declination 

 of the object observed read by small telescopes. If the 

 mirror can be moved at right angles to the meridian 5 

 from its central position, an object at the equator may be 

 followed for forty minutes, and other objects for a longer 

 period. Without this motion an object may be followed 

 for three or four minutes bv moving the eyepiece alone. 

 Clockwork may be applied to the mirror, or less easily to 

 the eyepiece. The focal length mav be increased almost 

 indefinitely if desired, and certain advantages will be thus 

 attained in the diminution in the defects of the object- 

 glass, although those of the reflector will not be affected. 

 If the telescope is to be erected at a great elevation the 

 advantages of the present plan are at once apparent. 

 Many nights of observation would be secured which 

 otherwise would be lost owing to the wind and cold. 

 The simplicity in the construction of the building would 

 be a great advantage, as a large dome in so exposed a 

 situation would be kept free from snow with much diffi- 

 culty, and might be a source of danger in winter storms. 

 If found impracticable to observe during the winter, it 

 would be possible to have a duplicate mounting below, 

 and remove the lens and mirror from one to the other. 



It is evident that the saving of cost would be very 

 great, not only in the observatory building aud dome, but 

 in the tube, observing chair, clockwork, &c. 



If a reflector could be constructed whose surface was 

 the portion of a paraboloid whose abscissa equalled that 

 of the focus, the instrument could be much simplified. 

 No object-glass would then be required, the reflector 

 taking the place both of mirror and lens. All the light 

 intercepted by the objective would thus be saved, and 

 but a single surface need be adjusted and corrected. 

 With the advance in mechanical methods this does not 

 seem wholly impracticable, especially with a mirror of long 

 focus. Since the final correction must always be made 

 by hand, it makes but little difference what is the exact 

 form of the surface. 



In any case it would be a great advantage that the 

 mirror could be reground, repolished, or resilvered with- 

 out moving it from its place. It would only be neces- 

 sary to place it horizontally, and the grinding machinery 

 could be kept permanently near it. If plane, the perfec- 

 tion of its form could also be tested at any time by setting 

 it on edge, and viewing the image it reflected by a colli- 

 mating eyepiece attached to the large telescope. Another • 

 method would be to place a heliotrope a few hundred 

 yards to the north or south of it, and the light from this 

 would form an excellent artificial star, available whenever 

 the sun shone. 



The greatest advantage is the rapidity with which ob- 

 servations could be made. No more time would be lost 

 in identification than with a transit instrument, so that a 

 large number of objects could be examined in the course 

 of a single hour. Any one who has worked with a large 

 telescope knows how much time is lost in opening and 

 closing the dome and in finding and identifying minute 

 objects. 



Let us now consider to what purposes a large telescope 

 mounted as suggested might be applied. 



1. Sweeping. For the discovery of new objects this 

 mounting presents especial advantages. It might be used 



for the detection of new double stars, of nebula*, of red 

 stars, or of objects having singular spectra, as planetary 

 nebuke, banded stars, and variables of long period. 

 Suppose that the field of view had a diameter of some- 

 what over one minute of time, and that a small motor 

 was attached to the mirror which would move it uniformly 

 over 5 in declination in this time, and then bring it 

 quickh back to its first position. The observer would 

 then have presented to him a series of zones 5 long and 

 one minute wide. The sweeps should overlap by a small 

 amount, so that the entire region could be covered in a 

 single evening. The observer could have a few seconds 

 rest between each zone, while the motion of the mirror 

 was reversed. If an object of interest was suspected, it 

 could be located by merely noting the time at which it 

 was seen. The right ascension would be given directly, 

 and the declination would be found by interpolation from 

 the time of beginning and ending the sweep. An exami- 

 nation of the object and a determination of its exact 

 location should be made on another evening. 



2. Measures of position. For many purposes positions 

 could be determined with this instrument as in a transit 

 circle. It would generally be better, however, to make 

 the measures differential, leaving the mirror at rest and 

 observing the transits of the object to be determined and 

 of two or more companion stars. The method of the 

 ring micrometer might be employed, 01 some modification 

 of that with inclined lines. In the latter case the zero of 

 position could be found by the transit of preceding stars, 

 by setting the reticule by a divided position circle, cr per- 

 haps better by keeping it in a fixed position, determining 

 the direction of the lines once for all, and applying a cor- 

 rection for the declination of the object observed. Stars 

 could be compared differing nearly a degree in declination, 

 as the eyepiece could be moved without danger of dis- 

 turbing the reticule. For the same reason the star could 

 be followed for three or four minutes, and its transit over 

 a great number of wires observed. It is here assumed 

 that the distortion produced by the mirror is not very 

 great. A slight distortion would do little harm, as it 

 would be the same for all stars of equal brightness. If 

 the stars differ greatly in brightness, the observer should 

 determine his personal equation between them in any 

 case, and the same operation would eliminate the effect of 

 the distortion. The large aperture of the instrument 

 would permit the observation of stars quite beyond the 

 reach of any meridian circle. The faintest asteroids 

 could thus be readily measured, and could probablv be 

 followed in many cases on successive evenings to their 

 stationary points. Zones of stars could be observed very 

 conveniently for the formation of charts or catalogues, 

 for the discovery of asteroids, stars with large proper 

 motion, &c. 



Probably the definition could not be sufficiently good 

 for the measurement of the closer double stars, but if 

 clockwork was attached, faint companions could be mea- 

 sured, or approximate positions of the coarser pairs de- 

 termined very rapidly. The positions of nebula* could 

 also be observed with a view to detecting their proper 

 motion. Stars having a large proper motion might be 

 observed, and the observations so arranged that any very- 

 large parallax would be detected. A similar search for a 

 large parallax of variable stars, short-period binaries, 

 minute planetary nebula, or stars having singular spectra, 

 might lead to interesting results. The argument that no 

 ordinary star is very near does not apply to such objects. 



3. Spectroscopy. The increased dimensions which 

 could be given to the spectroscope, and its steadiness, 

 would compensate in a great measure for a defect in 

 definition. By Zollner's reversion spectroscope the slit 

 might be dispensed with, and also the necessity of clock- 

 work. So many stars could be observed in a single even- 

 ing that systematic errors could be in a great measure 

 eliminated, and as the spectroscope would not be moved, 

 we should have a great assurance that the dev iations were 



