T. F. SMITH ON TRUE VERSUS FALSE IMAGES IN MICROSCOPY. 269 



the utilized diffraction beams of the structure in question." As I 

 understand Dr. Abbe, it means simply this : The image given 

 under the microscope may be a true enlarged image of the object, 

 or it may be the image of something totally different, and nowhere 

 does he give us a rule by which to distinguish the true from the 

 false ; so that, in spite of the increase of light in our objectives, 

 we are left in worse than " Egyptian Darkness " for the want of a 

 clear ray to guide us to a right interpretation of structure. 



It must be remembered that the first paper appeared in 1875, 

 at a time when microscopists were running after false methods of 

 illumination, and setting up diverse images of structure, produced 

 by throwing the light on the object from every direction except 

 the right one ; and I can understand an authoritative voice 

 declaring that the totally different results were due to the nature 

 of light itself, and not to bad manipulation, coming as a relief to 

 many minds weary of trying to formulate a system out of such 

 discordant elements. But since then has come the oil-immersion 

 objective and the oil-immersion condenser, throwing a flood of 

 light on the image not possible under the old methods ; and what 

 I cannot understand is that people should now revive the old 

 doubts. 



What I claim to-night is " Free Thought ' for microscopy, 

 and not dogma ; and I wish for a short time, by the aid of private 

 judgment only, to see how 7 the development of the same structures 

 under different apertures agrees with Dr. Abbe's " Diffraction 

 Theory." Let us begin, then, with our old friend Angulatum, 

 so frequently trotted out as a " shocking example " of depraved 

 exhibition of false structure, and see how far his appearance agrees 

 with this character. 



As Pleurosigma angulatum can be resolved into dots with a 

 •^ths in. of *64 N.A., there is plenty of space in which to watch 

 what changes take place in the structure between that and the 

 increase to 1*40 N.A., as provided by the new Apochromatic oil- 

 immersion. As a matter of fact, with the same illumination — 

 convergent light from a wide angled condenser — there is no change 

 whatever beyond increased sharpness of the image, which to me 

 proves one of two things. Either the whole of the diffraction 

 spectra have been taken up by the smaller aperture, or the 

 similarity of the image to the object does not depend upon all the 

 spectra being utilized by the objective. Of course, I am aware 



