372 OBJECTIVES, EYE-PIECES, THE APERTOMETER 



Now change the objective for the 16 mm. '3 N.A. (= f, but 

 with the same aperture). Nothing more is to be seen ; the most 

 dexterous manipulation cannot bring out a single fresh detail ; the 

 resolution is in no sense carried farther ; the cut suctorial tubes were 

 in fact, in our judgment, better seen with a lower power, while with 

 it all of course a smaller extent of the object occupies the ' field.' 



It can in fact be scarcely doubted that the picture presented 

 by the is a distinct retrogression in every sense compared with 

 that presented by the 1-inch when both are equally well made 

 and have equal apertures, viz. *3. But beyond all this, whatever 

 may be done by the 16 mm. *3 N.A. can be accomplished in an 

 equally satisfactory manner by removing the 1 2 eye-piece and 

 replacing it, with practically no other alteration, by [an 18 eye-piece ; 

 and still higher results can be obtained without the slightest detri- 

 ment to the image by using an eye-piece of 27. 



Not less interesting and convincing will it be to examine the 

 same object with a 12 mm. '65 N.A. (= ^-inch), and an A Zeiss 

 achromatic of *20 N.A. (= rds inch), using a 12 eye-piece. Those 

 who may still retain some conviction as to the value of ' low-angled 

 glasses to secure penetration ' can want no further evidence of its 

 entire fallacy than such a simple experiment affords. 



For those who prefer it, a true histological object may be selected. 

 We choose a portion of a frog's bladder treated w T ith nitrate of 

 silver, in which are some convoluted vessels, enclosed in a muscular 

 sheath which had contracted. 



This object is presented by photo -micrograph in figs. 7 and 8 of 

 the frontispiece. In fig. 7 the vessel in the frog's bladder is seen 

 by a Zeiss A *2 N.A. magnified 140 diameters. The object of the 

 photograph is to expose the fallacy which underlies the generally 

 accepted statement that low-angled glasses are the most suitable for 

 histological purposes. The assumption is founded on the fact that 

 the penetration of a lens varies inversely as its aperture, and it is 

 taken for granted that ' depth of focus ' will be obtained, not to be 

 secured by large apertures, and therefore it is taken for granted 

 that we are enabled to see into the structure of tissues. 



In examining the illustration (which will with advantage permit 

 the use of a lens) it will be seen that scarcely an endothelium cell can 

 be clearly seen. A sharp outline is nowhere manifest, because 

 the image of one cell is confused with the outlines of others upon 

 which it is superposed. We have seen that there is no perspective 

 proper in a microscopic image ; therefore it is better to use high 

 apertures in objectives, and obtain a clear view of one plane at one 

 time, and train the mind to appreciate perspective by means of focal 

 adjustment. 



It will be admitted that no clear idea of what an endothelium 

 cell is can be obtained from fig. 7. 



But fig. 8 (frontispiece) represents the same structure slightly 

 less magnified (x 138) by means of an apochromatic \ N.A. '65. 

 Here only the upper surface of the tube is seen ; but the endothe- 

 lium cells can be clearly traced, and a sharp definition is given to 



