132 TESTING THE MICROSCOPE. 



wire-gauze with a diameter of *6 to '7 mic. With the best of them 

 this diameter may he reduced even to '54 or *56 mic. Beneche's 

 No. 8 proved somewhat more powerful than his No. 9, as appeared 

 by the examination of organic test-objects ; still, the two numbers 

 always stand so close to one another that they may fairly be 

 regarded as one. A second No. 9 objective that we had an 

 opportunity of comparing, was between the two, its focal length 

 was 3 '15 mm., and the combined amplification with eye-piece 

 No. 2 about 340 ; the optical power was, however, somewhat less 

 than that of the No. 9 of the table. 



The most powerful objectives yet produced, among which are 

 the immersions of Hartnack, Gundlach, Zeiss, Beneche, &c., admit 

 of the resolution of meshes of from -4 to *32 mic. with the 

 eye-pieces generally used for observation, and with the most 

 favourable increase in the eye-piece amplification yield a further 

 addition of optical power up to about '3 mic. in diameter of the 

 meshes. Whether the best English objectives accomplish more, is 

 not known to us ; those we have had occasion to examine were at 

 most of equal power. 



A third remark which forces itself upon us, in comparing the 

 above table with the results of other testings, relates to the 

 absolute degree of performance. Measurements of images of wire- 

 gauze afford universally, and especially with objectives of large 

 angular aperture, somewhat larger figures for the limit of dis- 

 crimination than are obtained with organic test-objects. This might 

 have been expected ; the difference obviously arises from this, that 

 both the abnormal path of the rays, which the focal images of 

 the air-bubbles cause, as well as the kind of image-formation 

 in the Microscope dependent thereon, are unfavourable for the 

 objective. 



Organic Test-objects. 



Among the organic bodies, whose finer structural details a 

 good Microscope resolves only with a given amplification, the 

 small scales upon many insects, particularly upon the wings of 

 butterflies, and also the siliceous envelopes of diatoms, present 

 the most different gradations. The former were for a long 

 time employed almost exclusively as test-objects, and some are 

 even now used to a considerable extent ; the latter, which were 

 first suggested in England and are specially adapted for the 



