720 T. F. SMITH ON TRUE VERSUS FALSE IMAGES IN MICROSCOPY. 



that other appearances have been figured in this diatom, but as 

 they depend upon certain pencils of light being stopped out from 

 the back of the objective, I cannot regard this as a serious attempt 

 at elucidation of structure, and would as soon believe that an 

 actor's face could not be known to his friends because it assumes 

 different appearances under the limelight, as to believe that the 

 structure of Angulatum cannot be known because it is different 

 when certain of the diffracted rays are shut off. 



We come now to striated muscular fibre, of which I have brought 

 two examples here to-night, and have placed one under a |-in. 

 and the other under Zeiss' Apochromatic ^in. for your inspection ; 

 and I beg to say that it is with the optical appearances only, and 

 not the physiological properties I have to deal. Like the " rustic," 

 who, when asked if he understood the sermon, replied, " Would I 

 hae the presumption," it is not for me, in the presence of so many 

 medical gentlemen, to talk about structure, for the estimation of 

 which I ha ye received no special training, but I am concerned 

 with the truth or falsehood of the appearance of the structure, as 

 it affects all microscopic vision. When two specimens of the 

 same tissue, subjected to the same conditions of illumination, 

 quality, and aperture of the objectives, differ in their optical appear- 

 ance, it is obvious that change of structure, and not the effect of 

 light, accounts for the difference. The specimens are both teased- 

 out muscular fibre of pig, both from the same mounter, and have 

 been (I presume) treated in the same manner. The one under 

 the Jin. has the ordinary light and dark bands, with Krause's 

 membrane in the middle of the light ; the one under the Apochro- 

 matic ^in. is divided into equally placed discs, of which it is 

 impossible to tell which is Krause's membrane. Here also again 

 there is no difference of structure delineated by the objective of 

 the least aperture that will show it and the widest angled oil- 

 immersion, and there is the same characteristic divergence in both 

 specimens. I have examined them with a -lin. of "50 N.A. and 

 an oil- immersion of l - ±0, and beyond the difference between them 

 of faintness and sharpness of image there is no new development of 

 detail. Between the two extremes shown here there are more or less 

 indications in other specimens of imperfect cleavage transversely, and 

 it is on these diverse appearances that have been built up Hensen's 

 membrane, accessory discs, and all the other elaborate structure 

 which never had any differentiated existence except in the imagina- 



