The President's Address. By H. C. Sorhij, F.B.8., &c. 107 



1. Limit of the Powers of the Microscope. 



In treating this question I have no intention to enter into the 

 consideration of the hest form or arrangement of lenses to ensure 

 the least possible amount of spherical or chromatic aberration, nor 

 how far for the purposes of research it is desii'able to make a com- 

 promise between those practical difficulties which cannot all be 

 entirely overcome at one time, I shall assume that the instru- 

 ment itself is theoretically perfect, and consider only the limit of 

 vision due to the organization of our own eyes, and still more that 

 due to the physical characters of light. 



The visibility of a very minute object necessarily depends on a 

 number of different circumstances. If examined by transmitted 

 light it must either absorb sufl&cient to make the contrast between 

 it and the general field great enough for the eye to recognize, or it 

 must be of such a shape and of such a refractive power in relation 

 to the surrounding medium as to bend the light which passes near 

 the edges out of the general direction of the transmitted beam, so 

 as to give rise to a sufficiently dark and definite outline. In my 

 treatment of the question, I however assume that the character o£ 

 the object examined is in every respect such as would enable us to 

 see it, if it were not for difficulties of another kind. 



The purely physiological part of the question has not attracted 

 much of my attention, since I did not believe that the ultimate 

 limit of distinct vision would be found to depend on the con- 

 stitution of the eye. It may, however, be well to give a short 

 account of some experiments made by Dr. Eoyston-Pigott with 

 the view to determine the physiological limit, which he has kindly 

 communicated to me, and permitted me to employ, in order to 

 show that the above-named conclusion is justified by experiment. 

 He found that the smallest visual angle that he could ever dis- 

 tinctly appreciate was a hole Ij inch in diameter at a distance of 

 1100 yards, which corresponds to about 6" of arc. Tins visual arc 

 in a microscope magnifying 1000 linear would correspond to about 

 the three-millionth part of an inch. Some persons, however, affirm 

 that the smallest visible angle is 1', or ten times the above, which 

 would correspond to no oVot7 of an inch. If such be the case, the 

 eye could distinguish with a high magnifying power a much 

 smaller interval than the physical properties of light mil permit. 



Taking into consideration merely the swelling out of a minute 

 point of hght due to difiraction. Dr. Eoyston-Pigott thinks that the 

 limit of visibility must be from y-soVo o- to 20 oWo^ of an inch. 

 This, however, is not what appears to be the most important cha- 

 racter of light in limiting the power of the microscope for separatin"- 

 lines so near together that they may be obscured or their number 

 falsified by interference fringes. 



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