TRANSACTIONS OF SECTION A. B71 



nised. For all other kinds of astronomical pliotograpliy a fixed telescope is 

 admirably adapted; and so with all spectroscopic investigations, a little con- 

 sideration will show that the conditions under which these investigations can lie 

 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 coelostat 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 he 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. The 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 G 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. Bat with the plane mirror there will ba 

 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 favour- 

 able condition ibr good work, and the absence of motion during an observation, 

 except of course that of the plane mirror (which could be given by floiiting 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 mj' 5-foot 

 mirror was a long, imcertain, and expensive process. Now we have a method of 

 silvering mirrors that is certain, quick, and cheap. This takes away the cue 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 telescopa is to me the most beautiful ever made. Famili- 

 arity both in making and in using has only increased my admiration. With the 

 exception of the microphone of the late Professor 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, whilst 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 micro- 

 scope. 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, as it may possibly 

 be to the microscope. For a certain amount of light is necessary to produce sen- 

 sation in the eye. If this light is iusulKcient nothing is seen ; but owin? to the 

 accumulative 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 ses in the 

 telescope. All photographs, when closely examined, are made up of a certain 

 number of little dots, as it were, iu the nature of stippling, and it is a very inter- 



