THE MICROSCOPE. 



31 



anced, together with the general perfection of the workman- 

 ship. Now, in Wollaston's doublets, and Holland's triplets, 

 there are no means of producing a balance of the aberrations, 

 as they are composed of convex lenses only; therefore the best 

 that can be done is to make the aberrations a minimum; the 

 remaining positive aberration in these forms produces its pecu- 

 liar effect upon objects (particularly the detail of the thin 

 transparent class), which may lead to misapprehension of their 

 true structure; but with the achromatic object-glass, where the 

 aberrations are correctly balanced, the most minute parts of an 

 object are accurately displayed, so that a satisfactory judgment 

 of their character may be formed. 



" It will be seen by Fig. 15, that when a certain angular pen- 

 cil A O A' proceeds from the object O, and is incident on the 

 plane side of the first lens, if the combination is removed from 



V 



Fig. 15. 



the object, as in Fig. 16, the extreme rays of the pencil im- 

 pinge on the more marginal parts of the glass, and as the re- 

 fractions are greater here, the aberrations will be greater also. 

 Now, if two compound object-glasses have their aberrations 

 balanced, one being situated as in Fig. 15, and the other as in 

 Fig. 16, and the same disturbing power applied to both, that 

 in which the angles of incidence and the aberrations are small 

 will not be so much disturbed as where the angles are great, 

 and where consequently the aberrations increase rapidly. 



