•72 



THE AMEEICA^^ MONTHLY 



[April, 



The Physics of Yision with the 

 Compound Microscope.* 



It is my purpose this evening to 

 bring before you some of the results 

 of the studies of Professor E. Abbe, 

 of the University of Jena, Prussia, 

 which have shown us, that in portray- 

 ing the structure of objects, the diop- 

 trical action of the lenses in a micro- 

 scope, is very limited ; and that, ex- 

 cept in cases of very low amplification, 

 the performance of an objective is 

 dependent upon the physical nature 

 of light and the structure of the object. 



It may seem to you that the various 

 points are discussed in a superficial 

 manner, but it would be impossible 

 to treat the entire subject of the 

 physical action of the microscope in 

 one short paper ; I have endeavored, 

 therefore, to give a practical bearing 

 to my remarks, and to do this it has 

 been necessary to cover a large field. 



Moreover, the exhaustive treatise 

 of Prof. Abbe is not yet published, 

 and until this monograph is before us 

 it is not possible either to explain or 

 to fully understand a number of minor 

 points in his theory. Still, there can 

 be no doubt that his conclusions are 

 correct, and that to him belongs the 

 credit of having laid, after long and 

 patient research, the foundations of a 

 rational and true theory of the micro- 

 scope. 



There are objectives which work 

 when nothing but air intervenes be- 

 tween the front lens and the object. 

 Such objectives are known as dry- 

 objectives. The theoretical limit of 

 angular aperture for such objectives 

 is 1 80°. It is very evident that this 

 limit cannot be reached in practice ; 

 the greatest angular aperture that can 

 be obtained with good performance 

 with dry-objectives is not much above 

 1 10°. I cannot stop to explain why 

 this limit is so low, but as a few 

 makers have produced objectives for 

 which they have claimed angular 

 apertures far in excess of this figure, 



* Read before the New York Academy of 

 Sciences, during the "Winter of 1879-80. 



I will say that Professor Abbe's care- 

 ful researches have shown that a well- 

 corrected, dry lens cannot be pro- 

 duced, with the optical glass now 

 made, with an aperture above iro°. 



On the other hand, we have ob- 

 jectives which define only when some 

 fluid intervenes between the object 

 and the front lens. Such objectives 

 are known as immersion objectives. 

 The fluids used are selected on 

 account of certain physical properties 

 — density, refractive and dispersive 

 power — those most commonly used 

 are water and glycerin. 



In such objectives a ray of light 

 passing from the mirror, at an angle 

 of about 41°, will suffer total re- 

 flection at the upper surface, of the 

 cover-glass if the objective is a dry 

 one, but when an immersion lens is 

 used the ray passes directly on, and 

 enters the objective, only slightly 

 changed from its original direction. 



Thus we see that the available 

 angular aperture for dry-objectives, 

 for covered objects, is practically 

 limited by the angle of the total 

 internal reflection in glass. 



If the front lens is connected with 

 the cover by the fluid, it will be 

 observed that this fluid need only 

 possess a refractive index the same as 

 the glass in order to conduct the 

 incident rays in a perfectly straight 

 line from the object to the objective. 

 Thus, the immersion system makes 

 larger apertures available than can be 

 utilized with dry-objectives, besides 

 diminishing the loss of light by re- 

 flection. 



Quite recently objectives have 

 been introduced that are made on 

 the principle of homogeneous-immer- 

 sion, that is, the fluid intermedium 

 has a refractive and a dispersive 

 power the same as the glass of the 

 front lens ; the light therefore enters 

 the lens without refraction. This 

 system has rendered it possible to 

 greatly increase the angular aperture 

 objectives, and, so far as the resolu- 

 tion of minute structures is con- 

 cerned, the homogeneous-immersion 



