Nov. 13, 1884] 



NA TURE 



39 



confidently say they can, then it must be admitted that 

 nothing short of a revolution in observational astronomy 

 must result, to the enormous gain of astronomy. 



I speak relying entirely on my own work and experi- 

 ments, which I shall refer to in detail further on, and I 

 am strengthened in my opinions by what I hear has been 

 done in a similar direction elsewhere, though I have not, 

 except in one case, seen any of the actual work done. 



The possibilities that are thus opened out really border 

 on the marvellous. As has been already said by some 

 one else, a library may now be made, not of books full of 

 descriptions and figures, the accumulated work of many 

 men working many years, each on his own system, but of 

 pictures written on leaves of glass by the stars them- 

 selves. 



Such a work will mark an epoch in astronomy, and its 

 value increase as long as astronomers exist. No one can 

 doubt for one moment the importance of such a work, nor 

 the fact that, now it is possible, any delay in doing it will 

 be a direct loss to astronomy. How it is to be done — 

 whether by the slow process of letting it be done by the 

 disjointed efforts of many amateurs of astronomy, or by 

 being properly taken in hand and finished by united effort 

 and proper means in the course of a few years — remains 

 to be seen. 



I propose to make some suggestions as to the practical 

 part of this work in the selection of the best kind of tele- 

 scope and mounting, the methods of working, the work 

 to be done, and some other matters in connection. The 

 most important matter is no doubt the selection of the 

 best instrument to work with : of the two kinds of tele- 

 scopes now in use, the reflector seems to be the most 

 suitable for this work, though a reservation may be 

 made in favour of the refracting principle where large 

 fields on a small scale are required. Both kinds of tele- 

 hen of moderate dimensions, that is, not more 

 than iS inches aperture, are so nearly alike as optical 

 instruments that the chief distinction worth noting, 

 neglecting for the present one or two points where they 

 differ, and altogether such points as are rather matters 

 of individual prejudice on the part of the observer than 

 qualities or defects in the instruments, is that of cost, 

 the reflector being very much less expensive to make. 

 It is true that the refractor has been hitherto generally 

 considered the most satisfactory in use, and lias been 

 preferred when expense has not been a consideration of 

 importance. I think this may be rather due to the 

 greater care that is bestowed upon the more expensive 

 instrument, both in the making of the object-glass and 

 the mounting, than to any real difference that there is 

 between them. 1 he first cost of the raw material alone 

 differs immensely. For the reflector one disk of glass alone 

 is required, and if it is only properly annealed it need not 

 be optically pure. There is only one surface to work, 

 though it is of importance that this should be properly 

 figured, this is not a difficult matter, yet there is little 

 doubt it has often been very imperfectly done in many 

 so-called reflecting telescopes. 



For the refractor two disks of glass are required ; they 

 must be optically pure, and their first cost alone is more 

 the reflector, including the mount- 

 ing. These disks must be wrought on four surfaces to 

 proper curves, and time often spent afterwards in per- 

 fecting the object-glass; when this is done, the cost is 

 found to be so great that it is felt to be worth a costly 

 mounting. We cannot then be surprised that the better 

 made and mounted telescope should be chosen, but that 

 does not decide the question, Which is the best optical in- 

 strument ? Nor can this question be decided definitively, 

 because the images formed by each differ. If we look 

 with a reflector at a bright star, the image is seen as an 

 intensely bright point of light, dazzling to the eye if the 

 telescope is large, and we see rays or coruscations round 

 it of an irregular shape that are never steady. I think 



this effect is not due to the telescope, but is entirely sub- 

 jective, and caused by this extremely small point of light 

 exciting only a very small portion of the retina ; for by 

 proper precautions the light can be reduced, and these 

 rays and the dazzling effect got rid of. With stars less 

 bright it is not so pronounced, and on planets or objects 

 of sensible magnitude it ceases entirely. The image of 

 such a bright star in the refractor is quite of another kind : 

 it is seen as a small disk of light of sensible diameter sur- 

 rounded by the well-known system of diffraction rings and 

 outstanding colour. This disk of light though small, has 

 a different effect on the retina : it can be seen as a shape, 

 pretty steady and free from too much dazzling glare. It 

 is here that the refractor has such an advantage for 

 micrometrical work, permitting bisections to be made with 

 such precision. 



The adjustments of the object-glass are considered more 

 constant than those of the speculum, and though the 

 troubles attending the reflector are much exaggerated, 

 they have existed in the arrangements usually adopted. 

 For certain instruments such as the transit-circle, where the 

 connection between the optical axis and some part of the 

 instrument his to be maintained, the object-glass issuperior 

 to the speculum ; a tilt of the former that would not have 

 an appreciable effect on the position of the image of a 

 star would in the other displace this image twice the 

 amount of tilt. 



Both kinds have certain advantages, according to the 

 use they are put to, and it is really not of much conse- 

 quence which is the best instrument of this size. It is 

 when we begin to consider the effect of increased size and 

 all its attendant difficulties that the question of the suita- 

 bility of either for the purpose of photography has to be 

 answered. 



With the reflector increase of size means proportionate 

 increase in other qualities, in light-grasping power, in 

 defining, and in separating power. With the refractor the 

 greater absorption of light due to increased thickness 

 reduces the light-grasping power, and definition becomes 

 a matter depending not upon the optician but upon the 

 glass-maker ; the correction for colour, which even in 

 theory is approximate only, becomes more difficult, and 

 the defects due to the necessarily imperfect correction 

 become more apparent — and these two facts alone show 

 that as the refracting telescope gains in size it becomes 

 more and more unsuitable for photography. 



Moreover, when the aperture of the two kinds of tele- 

 scopes under consideration is the same, the focal length 

 of one must be something like twice that of the other, 

 and that means that the image is four times less bright, 

 and there does not seem to be any indication that the 

 focal length of refractors can be very much reduced. 

 This is only one part of the question, the next and most 

 important one is that of actual cost or difficulty of con- 

 struction. In the case of the refractor the preliminary 

 difficulty in getting the lumps of glass out of which the 

 lenses have to be made is so great that the increase of 

 the size beyond 30 inches seems at the present moment 

 very doubtful — they may reach 3-foot, or even 4-foot 

 aperture, but it is most unlikely : the cost alone, good or 

 bad, would be simply enormous, and such a size may be 

 for the present left out of consideration. With the re- 

 flector the case is entirely different : from what has been 

 said, it is easy to see that the gain by increase of size is 

 proportionate here, and that only mechanical difficulties 

 have to be met. Mirrors of glass covered or coated with 

 silver for the reflecting surface are now in existence of 

 3- and 4-foot aperture ; larger are in hand, and can be 

 made at a cost absurdly below the cost of even a possible 

 refractor : the only limit that I can see here is that of 

 glass, and the limit in this case stops not at 30 inches, as 

 with the refractor, but at something like 70 inches, and 

 that and nothing else of a constructive character prevents 

 the reflector being made much larger, and size is a great 



