CONSTRUCTION OF THE MICROSCOPE. 45 



mitted by the former than by the latter, when any 

 object is subjected to examination. In order to see an 

 object distinctly with an uncorrected lens, it is ne- 

 cessary to diminish the aperture so much, by the aid of 

 stops, as to interfere with the transmission of the amount 

 of light required to see the object perfectly. The greatest 

 angle of aperture of which a given lens is capable, will be 

 found by determining the greatest obliquity with which it 

 is possible for rays to fall upon the object-glass, so as to be 

 refracted to the eye-glass. Dr. Goring and Mr. Pritchard 

 contrived an instrument for ascertaining this, for any 

 given object-glass ; which instrument is fully described in 

 Dr. Lardner's useful little work on the microscope. 



A very perfect instrument for measuring the angle of 

 aperture, designed by Mr. Gillett, consists of two micro- 

 scopes, the optical axes of which may be adjusted to 

 coincidence. One of these is attached horizontally to the 

 traversing arm of a horizontal graduated circle, and is 

 adjusted so that the point of a needle, made to coincide 

 with the axis of motion of the movable arm, may be in 

 focus and in the centre of the field of view. The other 

 microscope, to which the object-glass to be examined is 

 attached, is fixed, and so adjusted that the point of the 

 same needle may be in focus in the centre of its field. The 

 eye-piece of the latter is then removed, and a cap with a 

 very small aperture is substituted, close to which a lamp 

 is placed. It is evident that the rays transmitted by the 

 aperture will pursue the same course in reaching the point 

 of the needle as the visual rays from that point to the 

 eye, but in a contrary direction ; and being transmitted 

 through the movable microscope, the eye will perceive an 

 image of the bright spot of light throughout that angular 

 space that represents the true aperture of the object-glass 

 examined. The applications of this instrument in the 

 construction of object-glasses are too numerous to be here 

 detailed: amongst the most obvious of which may be 

 mentioned the ready means it presents of determining the 

 nature, and measuring the amount of aberration in any 

 given optical combination. 



There is yet another source of inaccuracy which is more 

 mechanical than optical. All the lenses composing the 



