NOTES ON THE FUTURE OF THE MICROSCOPE. 

 By Eugene Schneider. 



A. Mechanical Improvement. — It is very difficult to make precise 

 suggestions as to mechanical improvement. The stands of the 

 diti'erent constructors are approaching a type, wliich in a measure, 

 tends to become classical. The initiative will in this development 

 apparently have to be taken by the scientists and industrials. 

 They will point out to the designers the defects of their instruments, 

 and will indicate the modifications which technical progress requires. 

 Yet we may specify one detail of improvement which might easily 

 be realised. For a long time all designers have adopted the 

 standard '' universal screw " for the objectives. Nothing analogous 

 has yet been done for the tubes in which slide the eye-pieces and 

 condensers. This is frequently a matter of inconvenience to the 

 microscopist, who possesses several instruments or who wishes to fit 

 eye-pieces and condensers of different styles into his microscope and 

 stand. 



B. Optical Improvements, a. Eye-pieces. — As regards the optical 

 parts, the microscopist, whatever his speciality, has all the necessary 

 instruments at his disposal. At the outside, one might wish for 

 eye-pieces of larger field for dissection or for the study of larger 

 slides. That however, would necessitate a larger tube diameter. 

 The field of the actual objectives is, moreover, of considerable curvature 

 already, and one would gain little by trying to carry the observations 

 to parts far away from the central portion. 



h. Objectives. — Abbe has shown that the definition of the micro- 

 scope is limited by diffraction effects, not on the edge of the objective, 

 but on the object. He has established that the definition — which is 

 frequently styled resolving power — is proportional to what he has 

 termed " the numerical aperture," which he defined by the expression : 

 numerical aperture = n sin u. There u is the semi-angular aperture, 

 that is to say, half of the apex angle of the cone of rays passing 

 through the object and admitted into the objective (Fig. 1), while n 

 is the refractive index of the medium surrounding the front lens of 

 the objective. More strictly expressed, n is the index of the least 

 refractive substance which is found between the object and the second 

 element of the front lens. 



But n is a function of the wave-length. In order to increase the 

 numerical aperture, and at the same time the theoretical range of 

 definition, we may hence increase u, or increase n, or decrease A 



Dry Systems of Objectives. — The object itself is immersed in a 

 medium of some refractive power, water, glycerin, Canada balsam, 

 etc. But a cushion of air is always left between the cover-glass and 

 the lens. As the refractive power of the air is taken as unit by 

 opticians, the n in that cushion has the value 1, and hence the 

 numerical aperature is equal sin u. In certain dry systems of 

 apochromatic objectives, the numerical aperture attains the value 0.95. 



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