THE PRESIDENT'S ADDRESS. 17 



This in general resembles their older form, except that the 

 distance between the optic axis and the limb has been increased, 

 so that complete rotation is given to the stage. The foot is an 

 equilateral tripod of 10-inch side, and its height is such that 

 when the instrument is placed in a horizontal position the optic 

 axis is ten inches from the table. The stage movements are 

 similar to Powell's No. 1, and the instrument is thoroughly 

 well made and sprung throughout. Leaving the microscopes, 

 we pass on to apparatus, and we have Mr. R. Smith's rocker 

 microtome. This is similar to the Cambridge rocker, but it is 

 fitted with a movable knife-holder, which allows the edge of 

 the blade to be placed at any desired angle to the cut. Another 

 ingenious arrangement permits the instrument to assume a 

 vertical position, so the cut can be made in a spirit trough. 

 We have also from America clay wicks for microscope lamps, 

 which are said to give a greater intensity of white light, and to 

 be without smell. Finally there is my own camera, which has 

 been already described here. 



We must now notice the improvement in coloured screens, a 

 very important branch of microscopy to which in the past it is 

 to be feared that sufficient attention has not been given. The 

 office of a screen in popular language is to improve the image 

 with apochromatics, strengthen the resolution, and at the same 

 time to soften the light when large illuminating cones are used. 

 They are of special service with cheap lenses (z.e., semi-apo- 

 chromats), because they remove the secondary spectrum, 

 making the lens as efficient as an apochromat. So markedly is 

 this the case that with equal apertures it is impossible to say 

 whether the objective on the nose-piece is an expensive 

 apochromat or a cheap semi-apochromat. In photomicrography 

 not only are the preceding remarks applicable, but also colours 

 difficult to photograph are rendered neutral. My reason for 

 dwelling at length again this year on the subject of screens is 

 not only because its importance is not so fully recognized as it 

 should be, but also because some improvements have been 

 effected during this past session. Mr. Lovibond, of Salisbury, 

 who has been both a member of this Club and also a Fellow of 

 the Royal Microscopical Society for twenty-nine years, has 

 given this subject much attention. He sent me a flashed glass 

 screen of peacock blue and a fluid screen of methylin blue, 



Journ. Q. M. C, Series II., No. 36. 2 



