352 THE president's address. 



powers from -J to J inch. Having personally examined a large 

 number of the following examples of this class I can speak 

 most highly of their performance. 



No. 6 Reichert (i) ... "82 N.A. ... O.I.* 13 7 



No. 7 Leitz Q) -71 N.A. ... 0.1.17-7 



8 m.m. Reichert (,4(j) ... "447 N.A. ... 0.1.18-6 



Of the low powers, 1 to 2 inch, one cannot speak so favourably ; 

 it would seem that, with the single exception of the Zeiss a.a. 

 (IJ) N.A. '19, O.I. 21, which is a very fine lens, they had been 

 altogether neglected, probabl}^ because medical students are 

 large buyers of these lenses, and any low power, a French 

 button, for instance, is " good enough for histological purposes." 

 But there is another large class of workers, especially in this 

 Club, to whom good cheap low powers, such as a 1 inch and 

 IJ inch, would be invaluable for the examination of pond life. 

 A good criterion for a li inch is the delineation of the cilia on 

 a vol vox globator. Of course a dark ground must be used. We 

 ought, therefore, to encourage as far as possible the semi- 

 apochromatization of low powers. 



With regard to eye-pieces nothing recently has taken place. 

 The most comfortable eye-pieces for general work appear to be 

 the 12-power compensating for the long tube, and the eight 

 power for the short, both being of the positive form. 



Projection eye-pieces present greater difiiculties ; the best, it 

 would seem, are constructed of a glass which crystallizes, and 

 those made of a durable kind of glass do not yield such sharp 

 images. Perhaps greater care is required in the technical 

 manufacture of these eye-pieces than is usually bestowed upon 

 them, for the fact remains that it is by no means easy to get a 

 good projection eye-piece. 



The substage condenser comes last, but by no means least, in 

 importance. We have all been told that the achromatization of 

 the substage condenser was an unnecessary refinement, but 

 that, however, has not been the opinion here, and the substage 



* O.I. means "Optical Index;" it is the numerical aperture of the 

 objective multiplied by 1000, and divided by the initial power. Tlie ideal 

 O.I. for a microscope objective is about 25. A true k inch of 60^ affords an fx- 

 cellent example. PowelTs apochromatic ^ has an O.I. of 23, and his ^^^ an 

 O.I. of 1'7 ; the smallness of the O.I. of the latter shows at once the fallacy 

 of its construction. The opposite error is exhibited in American | of 80°, 

 which some rears ago were thouorht a great deal of; they would have an 

 O.I. of 43 ("Journal R.M.S.," 1893, p. 12). 



