THE OPTICAL ARRANGEMENT. 



265 



therefore decidedly inferior to the modern achromatic systems. 

 Their practical applicability is now, however, limited to the magni- 

 fying-glass and the dissecting Microscope, and they are of consider- 

 able use up to an amplification of 60 100 linear; but it seems 

 probable that even for these purposes they will ultimately be 

 superseded by aplanatic systems, which are already much employed 

 for amplifications as low as 5 10 linear. 



For a comparison of the performances of the more recent 

 single lenses, doublets, &c., we give below a few of our observations 

 which were made in the same way as in the testing of the com- 

 pound Microscope. The focal lengths and object-distances (for 

 the cylinder lenses those corresponding to both positions) are given 

 in millimetres. 



From this table it is evident the doublets of Zeiss and Beneche 

 very nearly equal the performance of a compound Microscope with 

 equal amplification. The triplet of Zeiss is about equivalent to 

 Hartnack's No. 7 objectives and Beneche's No. 9 the latter tested 

 with an eye-piece amplification of 4 linear; it shows also the 

 transverse markings of Pleurosigma attemuttum by axial light with 

 at least equal distinctness. But this, however, is true only when 

 the object is exactly in the middle of the field of view ; in every 

 other position the image is valueless. Moreover, the vision is soon 

 fatigued by observing through a diaphragm of only *7 mm. in 

 diameter, which Zeiss applies above his triplets and highest-power 

 doublets. 



