Settembek 13, 1900] 



NATURE 



475 



excepting very long-continued observations, can be equally wei' 

 done by the fixed telescope. But there are some special lines 

 for which this form of research is admirably suited, such as 

 photographs of the moon, which would be possible with a 

 reflecting mirror of, say, 200 feet focal length, giving an image 

 of some 2 feet diameter in a primary focus, or a larger image 

 might be obtained either by a longer focus mirror or by a com- 

 bination. It might even be worth while to build a special 

 coelostat for lunar photography, provided with an adjustment to 

 the polar axis and a method of regulating the rate of clock to 

 correct the irregular motion of the moon, and thus obtain 

 absolutely fixed images on the photographic plate. 



The advantage of large primary images in photography is now 

 fully recognised. For all other kinds of astronomical photo- 

 graphy a fixed telescope is admirably adapted ; and so with all 

 spectroscopic investigations, a little consideration will show that 

 the conditions under which these investigations can be pursued 

 are almost ideal. As to the actual form such a construction 

 would take, we can easily imagine it. The large mirror mounted 

 as a ccelostat in the centre ; circular tracts round this centre, on 

 which a fan-shaped house can be travelled round to any azimuth, 

 containing all the necessary apparatus for utilising the light from 

 the large plane mirror, so as to be easily moved round to the 

 required position in azimuth for observation. In place of a fan- 

 shaped house movable round the plane mirror, a permanent 

 house might encircle the greater portion round the mirror, and 

 in this house the telescope or whatever optical combination is 

 used might be arranged on an open framework, supported on 

 similar rails, so as to run round to any azimuth required. Th° 

 simplicity of the arrangement and the enormous saving in cost 

 would allow in any well-equipped observatory the use of a 

 special instrument for special work. The French telescope has 

 a mirror about 6 feet in diameter and a lens of about 4 feet. 

 This is a great step in advance over the Yerkes telescope, and 

 it may be some time before the glass for a lens greater than 

 50 inches diameter will be made, as the difficulty in making 

 optical glass is undoubtedly very great. But with the plane 

 mirror there will be no such difficulty, as 6 feet has already been 

 made ; and so with a concave mirror there would be little 

 difficulty in beginning with 6 feet or 7 feet. The way in which 

 the mirror would be used, always hanging in a band, is the 

 most favourable condition for good work, and the absence of 

 motion during an observation, except of course that of the plane 

 mirror (which could be given by floating the polar axis and 

 suitable mechanical arrangements, a motion of almost perfect 

 regularity). 



One extremely important thing in using silver or glass mirrors 

 is the matter of resilvering from time to time. Up to quite 

 recently the silvering of my 5-foot mirror was a long, uncertain, 

 and expensive process. Now we have a method of silvering 

 mirrors that is certain, quick, and cheap. This takes away the 

 one great disability from the silver or glass reflecting telescope, 

 as the surface of silver can now be renewed with greater ease 

 and in less time than the lenses of a large refracting telescope 

 could be taken out and cleaned. It may be that we shall revert 

 to speculum metal for our mirrors, or use some other deposited 

 metal on glass ; but even as it is we have the silvered glass 

 reflector, which at once allows an enormous advance in power. 

 To do justice to any large telescope it should be erected in a 

 position, as regards climate, where the conditions are as favour- 

 able as possible. 



The invention of the telescope is to me the most beautiful 

 ever made. Familiarity both in making and in using has only 

 increased my admiration. With the exception of the micro- 

 phone of the late Prof. Hughes, which enabled one to hear 

 otherwise inaudible sounds, sight is the only sense that we have 

 been able to enormously increase in range. The telescope 

 enables one to see distant objects as if they were at, say, one- 

 five -thousandth part of their distance, while the microscope 

 renders visible objects so small as to be almost incredible. In 

 order to appreciate better what optical aid does for the sense of 

 sight, we can imagine the size of an eye, and therefore of a man, 

 capable of seeing in a natural way what the ordinary eye sees 

 by the aid of a large telescope, and, on the other hand, the size 

 of a man and his eye that could see plainly small objects as we 

 see them under a powerful microscope. The man in the first 

 case would be several miles in height, and in the latter he would 

 not exceed a very small fraction of an inch in height. 



Photography also comes in as a further aid to the telescope, 



NO. 161 I. VOL. 62I 



as it may possibly be to the microscope. For a certain amount 

 of light is necessary to produce sensation in the eye. If this- 

 light is insufficient nothing is seen ; but owing to the accumula- 

 tive effect of light on the photographic plate, photographs can 

 be taken of objects otherwise invisible, as I pointed out years 

 ago, for in photographs I took in 1883 stars were shown on 

 photographic plates that I could not see in the telescope. All 

 photographs, when closely examined, are made up of a certain, 

 number of little dots, as it were, in the nature of stippling, and 

 it is a very interesting point to consider the relation of the size 

 and separation of these dots that form the image, and the rods^ 

 and cones of the reckoner which determines the power of the 

 eye. 



Many years ago I tried to determine this question. I first 

 took a photograph of the moon with a telescope of very short 

 focus (as near as I could get it to the focus of the eye itself, 

 which is about half an inch). The resulting photograph 

 measured one two-hundredth of an inch in diameter, and when, 

 examined again with a microscope showed a fair amount of 

 detail, in fact, very much as we see the moon with the naked 

 eye ; making a picture of the moon by hand on such a scale 

 that each separate dot of which it was made corresponded with 

 each separate sensitive point of the retina employed when view- 

 ing the moon without optical aid, I found, on looking at this 

 picture at the proper distance, that it looked exactly like a real 

 moon. In this case the distance of the dots was constant, 

 making them larger or smaller forming the light or shade of the 

 picture. 



I did not complete these experiments, but as far as I went I 

 thought that there was good reason to believe that we could ia 

 this way increase the defining power of the eye. It is a subject 

 well worthy of further consideration. 



I know that in this imperfect and necessarily brief address 1 

 have been obliged to omit the names of many workers, but I 

 cannot conclude without alluding to the part that this Associa- 

 tion has played in fostering and aiding Astronomy. A glance 

 through the list of money grants shows that the help has been 

 most liberal. In my youth I recollect the great value that was 

 put on the British Association Catalogue of Stars ; we know the 

 help that was given in its early days to the Kew Observatory ;. 

 and the Reports of the Association show the great interest that 

 has always been taken in our work. The formation of a 

 separate Department of Astronomy is, I hbpe, a pledge that this^ 

 interest will be continued, to the advantage of our science. 



List of Large Telescopes in existence in 1900. 



Paris (Exhibition) 



Yerkes 



Lick 



Pulkowa . 



Nice 



Paris 



Greenwich 



Vienna 



Washington, U.S. 



Leander, McCormick 

 Observatory, Vir- 

 ginia 



Greenwich 



Newall's, Cambridge. 



Cape of Good Hope . 



Harvard . 



Princeton, N.J., U.S. 



Inches 



SO 



40 



36 



30 



29-9 



28-9 



28-0 



