300 THE PRESIDENTS ADDRESS. 



have gained some experiences which have proved interesting to 

 myself at any rate, and I trust that these may be either confirmed 

 or refuted by other and more expert observers. 



In what follows concerning optical appliances, reference will be 

 made principally to the use of a remarkably fine apochromatic 

 3 mm. oil immersion by Zeiss, of N.A. 1*43, In a few instances, 

 however, I have substituted for this a 2 mm. apochromatic of the 

 same aperture, kindly lent me by a friend. The illumination 

 employed has, for the most part, been that of an ordinary paraffin 

 lamp with half-inch wick, the light being rendered suitable for 

 my purpose by passage through a screen of signal-green pot glass, 

 the thickness of which, and consequently the intensity of illu- 

 mination, has been varied to suit the particular diatoms under 

 examination. Occasionally, however, I have so far modified my 

 practice as to substitute a methyl-green fluid screen for the simple 

 pot glass ; and to those who do not mind the extra trouble, I can 

 recommend this expedient, not only as furnishing a pleasanter 

 light, but also one of greater intensity, for the fluid screen 

 unquestionably passes more light than does the dry one, and the 

 flame is spectroscopically much more free from red rays. The fluid 

 screen, however, has its drawbacks, although these will doubtless 

 be overcome eventually, unless the screen itself is superseded by 

 something better : I refer to the fact that so far, at any rate, 

 no means has been discovered whereby the fluid (mainly glycerine) 

 can be retained within the tank which is supplied by the makers 

 for that purpose. The fluid inevitably gets heated in use, and, 

 expanding, finds its way out of the tank and gives rise to much 

 inconvenience. 



Whilst dealing with the general question of illumination, I 

 w^ould throw out a suggestion which will be found of value to 

 those who may be interested in the study of the most delicate 

 structures by the help of wide apertures and other optical 

 appliances of the most perfect type. I am convinced that even 

 those observers who duly appreciate the value of malachite-green 

 screens for the illumination of objects in the microscope, pay far 

 too little attention to the quality of the difliised light of the 

 room in which they work ; the consequence being that the retina 

 speedily loses that exquisite sensitiveness which it acquires when 

 the quality and quantity of the light of the work-room are 

 adapted to the circumstances. 



