ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 289 



I was myself caught witli this plausible fallacy some years ago, and 

 lost a good deal of ground before I discovered the error. This fallacy is 

 an exceedingly difficult one to confute in an article such as this, because 

 it does not consist solely of one error, but is, in reality, a whole cluster 

 of errors. 



To illustrate one of these errors, I take two quarters, one wide, the 

 other low-angled, both of the same power, and both thoroughly well 

 corrected. I, the demonstrator, allow the demonstratee to select any 

 object out of my cabinet, or bring one of his own ; this object is then suc- 

 cessively examined by each lens, properly illuminated by a condenser, and 

 by the same eye-piece; the unanimous testimony of the demonstratees 

 being that the observations are much more satisfactory with the wide- 

 augled lens. 



I then ask the demonstratee to fix on some definite point in the object 

 illustrating the superior penetrating power of the low-angled lens. 



The object is re-examined, the objectives are changed a score of 

 times ; but the result of it all is, that they say, they thought there 

 would have been more difierence ; but, practically, there does not appear 

 to be any. I have devoted a great deal of time to this question, and 

 have gone carefully over these experiments myself, and consequently 

 know what must be the verdict of every impartial observer. 



The explanation is as follows : — Let us suppose that a section of 

 tissue is the object,* and the test is to trace the course of a vessel in it. 

 Where no special difficulty arises the one lens is as good as the other,f 

 but the moment the vessel gets involved in similarly coloured or other 

 tissue, the increased resolving power of the wide-angled lens makes itself 

 felt, and at once differentiates the structure, which the narrow-angled 

 lens fails to do. 



The difference of focal depth might form an element for considera- 

 tion if the lens were rigidly fixed at a certain distance from the object ; 

 but there is such a thing as a fine-adjustment, and by means of it the 

 observer is able to trace the course of the vessel by the wide-angled lens, 

 and without effort or thought to direct the movement of the lens ; there- 

 fore, the question of depth of focus assumes more of a theoretical 

 objection than a practical one. 



The question will be asked, What has all this to do with condensers ? 

 The answer is, that histologists invariably use narrow-angled lenses. A 

 plane mirror when used with diffused daylight near an ordinary window 

 gives a cone of illumination; the angle of this cone varies with the 

 diameter of the mirror and its nearness to the object, say from 10° to 

 30°. In no instance of mirror illumination, either plane or concave, 

 would I expect to find 30° exceeded, and such an angle as that would 

 never be reached by the average histological Microscope. 



Therefore we can see that a histologist would, perhaps, be able to fill 

 his low-angled inch, and to inadequately fill his low-angled 1/4. The 

 1/4 he would not be able to fill enough to develope any spherical aber- 

 ration it might have, unless, indeed, the lens was execrably bad. 



* Histologists invariably choose this or some similar object. They fight shy of 

 diatoms, because they know the superior defining power of the wider-angled lens 

 would be more apparent. The diatom woixld exhibit the difference of focal depth 

 better than any other histological object. 



t Not strictly speaking, for a narrow-angled lens never gives such a good image 

 as a wide one. 



