president's address. 37 



In almost every astronomical instrument the loss of light due 

 to numerous surfaces of lenses and mirrors rapidly limits their 

 efficiency. We endeavour to make up for this loss by increasing 

 the dimensions of the first light collecting element of the 

 instrument, such as the mirror or object glass in telescopes. 



So far, however, with a few exceptions, it has been found there 

 is a limit which appears to have been rapidly appproached in some 

 of the largest telescopes recently constructed. 



In the case of refractors of 26 to 30-inch aperture, the lenses of 

 the object glass become so thick as not only to absorb a serious 

 proportion of the light it grasps, but its weight brings about 

 flexure of the glass itself, and strain which greatly interferes 

 with good performance. It now seems likely, if larger apertures 

 are attempted, some supports for the centre of the lenses must be 

 devised, which of course will to some extent diminish the aperture 

 it was sought to increase. 



Although the same class of objections do not lie against 

 reflecting telescopes, yet it does not at present seem profitable to 

 go beyond the dimensions now used — say 6 feet diameter at the 

 extreme, for no reflector of even 4 feet aperture exists that is free 

 from serious defects arising from weight and difficulties of 

 preventing flexure, detrimental, if not ruinous, to good perform- 

 ance in the mirrors and other parts of the instrument. So far, 

 then, I think we may state generally that increase in size of 

 telescopes has not been followed by that increase of usefulness 

 which largely increased cost naturally demands. It has been 

 said the most important part of the astronomical telescope is the 

 observer's eye ; this cannot be made more perfect, still its 

 imperfections are a bar to further advance in some directions. 

 No two eyes are alike, and no two see an object exactly alike, and 

 so there is always an outstanding doubt between results of two 

 or more observers. Again, there is that nerve personality which 

 is a great drawback in observational astronomy, and is due to 

 ditferent individuals possessing nerve current velocity of difi'erent 

 degrees. 



The question of stability of astronomical instruments is of the 

 utmost importance, but we know now there is no such thing as 

 stability on the earth's surface, and we are not only subject to 

 incessant tremors but actual movements, which may be periodic 

 and oscillating, or pei-manent and sliding. To ascertain these 

 movements and provide against their results upon celestial 

 measurements forms a large part of the practical astronomer's 

 work. 



With the astronomical spectroscope loss of light through 

 multiplication of surfaces and absorption of the material of which 

 the prisms are made is the great difficulty, except where there 

 is abundance of light, as in the case of the sun. For this reason 

 spectroscopic observations of the smaller stars and nebulae is 



