304 THE president's address. 



exact nature of the fluid or gaseous matter in these spaces I 

 am quite unable to state ; but I imagine that in the process of 

 fusion the realgar is decomposed, and that some product of the 

 decomposition of enormously high refractive index remains in 

 the spaces referred to. 



However this may be, it should be mentioned as a fact 

 worth notice, that there is no difficulty in resolving the valve of 

 AmphipleurajjeUu.cida, to which I have referred, with an ordinary 

 Y^^th-inch achromatic oil immersion of 1-3 N.A. by the help of 

 direct light and a |-cone from a dry apochromatic condenser. 



I have even resolved this particular valve with a water 

 immersion ^ of 1"26 N.A., made for me twenty years ago by 

 Mr. Thomas Powell, and this with only very slight obliquity of 

 illumination. I have not yet had an opportunity of trying the 

 experiment, but I feel convinced that if a larger cone of aplanatic 

 light could be used to illuminate this valve, my |-inch would 

 resolve it with direct light. 



The slides I already possess, mounted in realgar, have been 

 prepared by Messrs. Thum. I have specimens of A. jyelhicida 

 and a few Nitzschias, and also specimens of F. angulatum and of 

 SurireUa gemma. They are all spread slides, and the diatoms are 

 certainly shown with exquisite clearness. There is, however, 

 another objection to the employment of realgar, in addition to 

 the serious danger to life from the inhalation of noxious fumes 

 and the intensely yellow colour of the medium, to w^hich I have 

 already referred. The temperature necessary for the fusion of 

 realgar is stated to be so high that it is impossible to mount a 

 selected slide in this medium ; for there is no adhesive substance 

 yet known that will resist such a temperature, and consequently 

 any trouble that may have been taken to arrange diatoms on 

 the cover-glass is almost certain to prove fruitless. 



Another medium which has given me satisfactory results of 

 late, and for the knowledge of which I am again indebted to the 

 kindness of Mr. Morland, consists of a combination of Piperine 

 with Bromide of Antimony. This mixture is prepared by com- 

 bining three parts by weight of Piperine and two of Antimony 

 Bromide : it must be very gently fused over a spirit lamp, care 

 being taken not to raise the temperature more than is necessary, 

 or the mixture becomes charred and discoloured and consequently 

 worthless. After the diatoms have been spread on the covei' 



