28 



THE MIOEOSOOPE. 



"The direction of the aplanatio pencils appears to be scarcely 

 affected by the differences in the thickness of glasses, if their 

 state as to color is the same. 



" One other property of the double object-glass remains to 

 be mentioned, which is, that when the longer aplanatic focus 

 is used, the marginal rays of a pencil not coincident with the 

 axis of the glass are distorted, so that a coma is thrown out- 

 wards; while the contrary effect of a coma directed towards the 

 centre of the field is produced by the rays from the shorter 

 focus. These peculiarities of the coma seem inseparable at- 

 tendants on the two foci, and are as conspicuous in the achrom- 

 atic meniscus as in the plano-convex object-glass. 



"Of several purposes to which the particulars just given seem 

 applicable, I must at present confine myself to the most obvious 

 one. They furnish the means of destroying with the utmost 

 ease both aberrations in a large focal pencil, and of thus sur- 

 mounting what has hitherto been the chief obstacle to the per- 

 fection of the microscope. And when it is considered that the 

 curves of its diminutive object-glasses have re- 

 quired to be at least as exactly proportioned as 

 those of a large telescope to give the image of a 

 bright point equally sharp and colorless, and 

 that any change made to correct one aberration 

 was liable to disturb the other, some idea may 

 be formed of what the amount of that obstacle 

 must have been. It will, however, be evident 

 that if any object-glass is but made achromatic, 

 with its lenses truly worked and cemented, so 

 that their axes coincide, it may with certainty 

 be connected with another possessing the same 

 requisites and of suitable focus, so that the 

 combination shall be free from spherical error 

 also in the centre of its field. For this the 

 rays have only to be received by the front glass 

 B (Fig. 14) from its shorter aplanatic focus F", 

 and transmitted in the direction of the longer 

 correct pencil F A of the other glass A. It is 

 desirable that the latter pencil should neither 

 Fig. 14. converge to a very short focus nor be more than 



