Jan. 22, 1885] 



NATURE 



271 



graphy where so much depends on the power of the tele- 

 scope to bring together as much light as possible 

 on the surface of the sensitive plate. Apart from 

 this great loss of light there would be enormous 

 difficulties in making such a telescope of even three- 

 foot aperture, indeed, I am very doubtful if it could be 

 done, there is the difficulty of keeping the different mirrors 

 free from flexure and in proper adjustment, there is the 

 fact that the form of mounting that must be used to carry 

 the ponderous mirrors would be that most unfavourable 

 to the gobd performance of the whole as a telescope in 

 regard to the atmospheric disturbance due to the mount- 

 ing : and last, though not least, the position of the 

 external plane mirror would be so exposed that it would 

 not stand many nights' work ; with the flat mirrors of a 

 Newtonian telescope one has much difficulty, as a slight 

 rise in temperature will dew them at once, and under 

 ordinary circumstances they become very soon so dull 

 that they require re-silvering many times more frequently 

 than the large mirror. Certainly the large plane mirror 

 would conserve its heat better than the small flat of a 

 Newtonian, but from the exposed position it would 

 occupy, it would certainly be a source of continual 

 trouble. There is only one good thing in such arrange- 

 ments, and that is that the observer has not to follow the 

 eye-piece, which only rotates, and does not change its 

 position. For general observational work this becomes 

 of importance. For comet-seeking, for which 1 believe 

 this telescope was first used, it is difficult to imagine a 

 more suitable arrangement than that brought again to the 

 notice of astronomers by M. Hermite in L'Astronomie, 

 October 1884, though his proposition, to dispense with a 

 tube or to use a fixed one, would make a difficulty at the 

 eye-end, where the image would rotate, as it would in the 

 case of a fixed telescope with a mirror moving in front, 

 after the manner of a siderostat. For photography all 

 those latter forms of telescope are not admissible ; even 

 for large fields, when a refractor specially made was used, 

 it would be better to use it as a simple equatorial than to 

 lose the light by two additional reflections. Considering 

 carefully the different reflecting telescopes enumerated 

 above, there does not appear to be anything that can be 

 more simple than the Herschelian, and nothing more 

 )le, judging from what has been done, than the 

 Newtonian ; nor does there seem anything in any other 

 form that offers greater advantages than these, either on 

 the grounds of simplicity, easy manipulation, possible 

 increase of size, and, what is of vital importance, small- 

 ness of first cost ; it is on one or the other that I should 

 entirely rely as the photographic telescope of the future ; 

 whether the Herschelian form would be better in prac- 

 tical use than the Newtonian, or, rather, whether the 

 reflecting surface could be made as good in this case, 

 would only be shown by actual trial ; if it could then, for 

 the reason already mentioned, the image would be the 

 best, and the best kind of telescope for the purpose of 

 photography would be found. 



In the Newtonian, as has been said, the plane mirror' 

 is only used to bring the rays, that would form the image 

 otherwise in the centre of the tube, out at the side, but as 

 the object is not to be viewed, but photographed, the 

 plate can be placed in the proper place to receive those 

 images direct from the large mirror, as was done by Dr. 

 De la Rue when he first used the reflecting telescope for 

 photographs of the moon. 



There are some difficulties in getting a proper super- 

 vision of the exposure, but these are not insuperable. A 

 mounting for the Newtonian reflector pure and simple 

 would be equally suitable for the Herschelian, so that if 

 it were decided to make a large telescope, no danger 

 would be run that success would not be certain ; if the 

 Herschelian gave such excellent results, as I think might 

 be fairly expected, so much the better, if it did not, the 

 telescope that has already shown its capacitv would 



simply remain what it is now — the only telescope 

 suitable for photography on such a scale as can be 

 really useful. 



As to the way in which such a telescope as I here 

 contemplate, that is, a reflector of from 5 to 8 feet aper- 

 ture, should be mounted, there would be a certain safety 

 in following the plan I have found so good with 3 foot, 

 with such mechanical alterations as the use of water in 

 place of mercury for the floating medium would render 

 necessary. The general principles, I believe, are correct 

 as regards the conditions that affect the performance of 

 the telescope as an optical instrument. 



The duty of the observer would now be entirely limited 

 to seeing that the image fell always on the same place on 

 the plate during exposure, a duty that is easily described, 

 but not so easily done. For this purpose he must have 

 such optical arrangements that he can from the ground 

 watch the position of the image of a star anywhere near 

 the object to be photographed in its relation to a cross 

 wire attached to and moving with the sensitive plate, so 

 that if, from the many causes that can produce a shift of 

 this star and of the image on the plate, there is a slight 

 movement, he can at once correct it. The telescope would 

 work entirely in the open air under the most favourable 

 conditions and without any disturbance from the body of 

 the observer, as he would not be near the high end of 

 tube. The large mirror would be protected from dew by a 

 slight covering round the skeleton tube, and have an 

 apparatus to cover it up quickly, and so be in the best 

 condition to keep its polish, and with the absence of a 

 small mirror and its trouble at the high end of the tube, 

 simplicity would be followed to its fullest extent without 

 the sacrifice of one essential point. 



Such a telescope would be capable of giving photo- 

 graphs of all the nebula;, with exposures of from 30 to 60 

 minutes, of the various clusters, and of certain selected 

 parts of tke heavens, and this should be for some years 

 its chief work. About the value of such a work it is 

 quite unnecessary to speak — to show that it can be done 

 is quite enough. 



In thus giving my opinion as to the best kind of tele- 

 scope to use for this most important part of astronomical 

 photography I place it first for its importance. That 

 much could be done by a smaller instrument, or, rather, by 

 many smaller instruments, of a most valuable character, I 

 have not any doubt. It is quite possible now, by means 

 of photographic lenses, to take stellar photographs that are 

 of great value ; and any equatorial reflector, and many 

 refractors, if they have driving apparatus of fair quality, 

 could be most usefully employed in photography, and that 

 without any more knowledge of the art of photography 

 than could be learnt in a few minutes ; by taking photo- 

 graphs of a small portion of the sky that could be identi- 

 fied, and working entirely at that, the amateur astronomer, 

 with any aperture over 6 or 8 inches, could make a mono- 

 graph that would be good for all time, and his results 

 would not be the mere expressions of impressions on his 

 mind through his eye, but would he visible ones that 

 would speak for themselves as to their value. In all 

 departments of stellar photography, excepting of course 

 absolute positions, I think that photography is at once 

 available. It is remarkable that the silver-on-glass re- 

 flector has proved itself to be capable of practically un- 

 limited increase in size and to be so well fitted for pho- 

 tography at the same time that the photographic process 

 has been brought to such a state of perfection, especially 

 in this country, the home, if not the birthplace, of the 

 reflector. At the present moment a gigantic stride in 

 advance is to be made with certainty of success, and that 

 at a cost that is insignificant compared to the results that 

 must come. Let us hope some one who can hasten this 

 step will come forward ; if one cannot, many must, for it 

 should not be delayed. 



A. Ainslie Common 



