BY A. B. BIGGS. 19 



which the instrument is capable of being developed. This is 

 the question which it is natural to ask, and which I have been 

 asked repeatedly. On this account I have thought it might 

 be deemed an interesting question to discuss. 



I may state at starting that doubtless there is a limit, and 

 moreover, I believe that limit is already nearly, if not quite, 

 attained. To prepare the ground for a clear understanding of 

 my reasons, I must very briefly refer to the fundamental 

 principles of the telescope. A luminous body, such as a star, 

 emits rays diverging in all directions in straight lines. Of 

 course the greater the distance the more these rays are 

 spread ; that is, the light is diminished in intensity, and that 

 in proportion to the square of the distance. Of the total light 

 emitted that received by the eye is that fraction which the 

 pupil bears to the whole sphere of which the radius equals the 

 distance. These rays may be so enfeebled by distance that 

 the object ceases to be visible. Now, if we can gather a 

 sheaf of these rays that would otherwise pass by us unper- 

 ceived, and bend them inwards until they enter the pupil of 

 the eye, we evidently increase the visible luminosity in that 

 proportion. This is the function of the object glass — i.e., the 

 large glass at the outer end of the telescope — or the speculum 

 of the reflecting telescope (See Fig 1). 



Of course, the larger the glass the more it can gather. 

 As the rays from every point of the object are brought to a 

 corresponding point at the focus of the lens, a small image 

 of the object is formed. This image is magnified by the 

 eyepiece, which is really a microscope. But whether we mag- 

 nify this image less or more, we have only the same amount 

 of light to deal with, viz., that which is grasped by the object 

 glass ; and the more this light is spread out by magnifica- 

 tion the more it is enfeebled. This magnification is there- 

 fore limited by the capacity or size of the object class — 

 (other things being equal). In this respect the Telescope 

 and the Microscope stand on a very different footing. We 

 cannot, of course, increase the actual luminosity of a celestial 

 object in the smallest degree. All we can do is to grasp as 

 much as possible of the light actually emitted. With the 

 microscope, on the other hand, we can, by means of conden- 

 sers, and the employment of suitable sources of light, 

 increase the illumination of the object indefinitely up to 

 the required amount of amplification. Hence it is evident 

 that increase of telescopic 'power is to be sought in the enlarge- 

 ment of the light gatherer — the object glass. But this is 

 where the trouble comes in. The difficulty of accurately 

 figuring a large object glass, or speculum, increases enor- 

 mously out of proportion to its dimensions. If every point 

 of its active surface does not refract or reflect its ray to its 

 exact point in the focus, such defective part is worse than 



