172 



PRACTICAL TEEATISE ON THE MICROSCOPE. 



the definition produced by an achromatic compound micro- 

 scope will depend upon the accuracy with which the aberra- 

 tions, both chromatic and spherical, are balanced, together 

 with the general perfection of the workmanship. Now, in 

 WoUaston'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 

 peculiar 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. 119, that when a certain angular pencil, A O A, pro- 

 ceeds from the object, O, and is incident on the plane side of 

 the first lens, if the combination be removed from the object, 

 as in fig. 120, the extreme rays of the pencil impinge on the 



Fig. 119. 



rig. 120. 



more marginal parts of the glass, and as the refractions 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. 119, and the other as in fig. 120, 

 and the same disturbing power applied to both, that in which 

 the angles of incidence and the aberrations are smaU, will not 



