president's address. 215 



economy of power,"* and pointing out that when depth dimen- 

 sion is needed in observation, the low or moderate powers are 

 necessary, he then affirms that " no greater aperture should 

 (therefore) be used than is required for the effectiveness of 

 these powers — an excess in such a case is a real damage. "t In 

 truth, it appears almost as an axiom of the diffraction theory of 

 microscope vision, that we should employ the full aperture 

 suitable to the power used. That, in short, to over-aperture a 

 given power in an object-glass is to ruin it. 



Curiously enough this appears to be recognized in a broad 

 sense in one part of the book in question,;]: but in an earlier 

 page§ the author recommends now what he calls the " American 

 thread" as distinct from the "society screw" for low-power 

 objectives, because its larger diameter admits of back lenses to 

 the objectives of greater diameter, " and thus offers certain 

 advantages," while it is said that the larger lenses are easier to 

 make, and the real curvatures are, therefore, approximated 

 more easily to the calculated curvatures. In other words, we 

 are recommended noiv to employ for low powers a gauge greater 

 in diameter than that allowed by the society screw, so as to be 

 able to employ back lenses of greater diameter. 



This, in effect, means that we should give greater apertures 

 to low powers — apertures, that is, greater than can bo obtained 

 ivithin the diameter of the society screw. 



Now there was a time when these lenses were experimentally 

 sought, but it was before either oil immersion or apochromatic 

 days. The matter was first mooted in 1879, and the next year 

 a screw or gauge was brought out by a Mr. Butterfield, having 

 a wide diameter, so as to lend itself to a great back lens, and 

 an absurdly large aperture to low-power object-glasses. 



Always desirous of obtaining the advantage of any improve- 

 ment, and deficient then in the knowledge we now possess, I 

 induced Messrs. Powell and Lealand to make me a two-thirds 

 O.G., with as great a back lens as the society screw would 

 admit. I have that glass now, and its over-apertured condition 

 is patent. 



What then must it be with an enlarged diameter for the 



* "Journal R.M.S.," Series ii,, Vol. ii., p. 304. 



t Ihid. 



X " The Microscope," p. 56. 



§ Ihid., p. 49. 



