390 



NATURE 



\August 25, 



be saved, and but a single surface need be adjusted and corrected. 

 With the advance in mechanical methods this does not seem 

 wholly impracticable, esjiecially 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 without moving it from its 

 place. It would only be necessary to place it horizontally, and 

 the grinding machinery could be kept permanently near it. If 

 plane, the perfection of its form could also be tested at any 

 time by setting it on edge, and viewing the image it reflected by 

 a collimating eyepiece attached to the large telescope. Another 

 method would be to place a heliotrope a feiv 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 observations 

 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 nebulce, of red stars, or of objects having 

 singular spectra, as planetary nebula;, banded stars, and variables 

 of long period. Suppose that the field of view had a diameter 

 of somewhat 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 quickly 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 examination of the object and a deter- 

 mination 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, or 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, or perhaps 

 better by keeping it in a fixed position, determining the direction 

 of the lines once for all, and applying a correction for the de- 

 clination of the object observed. Stars could be compared dif- 

 fering nearly a degree in declination, as the feyepiece could be 

 moved without danger of disturbing the reticule. For the s.ime 

 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 probably be followed 

 in many cases on successive evenings to their stationary points. 

 Zones of stars could be observed very conveniently for the for- 

 mation 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 measured, or approximate 

 positions of the coarser pairs determined very rapidly. The 

 positions of nebulae could also be observed with a view to de- 

 tecting 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 nebulce, or stars having singular spectra, might lead to 

 interesting results. The argument that no ordmary 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 compen- 

 sate in a great measure for a defect in definition. By ZoUner's 

 reversion spectroscope the slit might be dispensed with, and also 

 the necessity of clockwork. So many stars could be observed 

 in a single evening 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 deviations 

 were real. Of the 6000 nebulae hitherto discovered we know 

 nothing of the spectrum of more than 300 or 400, while the 

 observation of all the others with a large horizontal telescope 

 would not be a very formidable undertaking. It would also be 

 interesting to observe the spectra of all the clusters. It is 

 possible that some may consist of stars having singular spectra, 

 or even of disconnected nebulous masses, in fact forming clusters 

 of planetary nebulce. The interesting discovery by Dr. Copeland 

 that Burnham's double nebula in Cygnus is gaseous, shows the 

 same tendency to aggregation in these bodies as in stars. Ob- 

 servations of the spectra of all the red stars and variables would 

 also probably lead to interesting results. 



4. Photometry. Should the instrument be devoted to photo- 

 metry numerous problems suggest themselves. Variable stars 

 could be observed near their minimum when too faint to be 

 identified with an equatorial without great loss of time. Faint 

 stars in zones or faint companions to bright stars could be 

 measured very rapidly. The relative light of all the asteroids 

 would be an interesting problem. Many coarse clusters appear 

 to consist of stars of nearly equal brightness. Their light com- 

 pared with their distances apart might aid our study of their 

 formation. Another useful investigation would be to measure 

 the brightness of all the nebulce. 



In the application of physics to astronomy doubtless many 

 other problems will suggest themselves. Thus no satisfactory 

 results have been obtained in the attempt to measure the heat of 

 the stars with the tasimeter. The use of this instrument would 

 be vastly simplified if it was placed on a solid pier near the 

 ground, was not moved during the observation, and could be 

 perfectly protected from other changes of temperature than those 

 which it was intended to measure. 



As either of the problems proposed above would occupy the 

 time of a telescope for at least one year, it is obvious that there 

 could be no difficulty in keeping such an instrument occupied 

 indefinitely. 



The horizontal mounting is especially adapted to an elevated 

 position, and would ;'permit the use of a telescope where an 

 equatorial mounting would be quite impracticable. On the 

 other hand, to an amateur, or for purposes of instruction, an 

 instrument which could be set quickly from one object to another, 

 and where the observers need not be exposed to the cold, would 

 offer many advantages. The impossibility of observing far from 

 the meridian would be less important with a large instrument, 

 where the number of objects to select from is very great. 



There are certain purposes to which this mounting could not 

 be advantageously applied. The study of close double stars and 

 other objects requiring long examination and very perfect defini- 

 tion could be better left to other instruments. The sun, inoon, 

 and planets can also generally be better observed off the meridian. 

 If, however, the entire time of an instrument can be employed 

 to advantage, and it can collect several times as much material as 

 an instrument of the usual form, it is no evidence against its trial 

 that there are certain problems to which it cannot be advan- 

 tageously applied. 



The working force required for such an instrument should 

 consist of at least one observer, an assistant to record, and a 

 number of copyists and computers to prepare the working lists, 

 reduce the observations, prepare them for the press, and read 

 and check the proof-sheets. A large volume of valuable obser- 

 vations could thus be produced every year, which would require 

 at least double the time and money to produce by the same 

 telescope mounted equatorially. The difference in the amount 

 of work will be evident when we compare the number of obj ;cts 

 observed with a transit instrament per night, with those observed 

 with an equatorial. A hundred objects in various declinations 

 might be examined in a single evening, while it is seldom that 

 the same number could be identified and measured by an equa- 

 torial in a week. 



