THE president's ADDRESS. 303 



But, unfortunately, quinidine has its faults; for up to the 

 present time no one has succeeded in preventing the crystallisation 

 of this substance. Occasionally a slide will remain perfectly trans- 

 parent for years. My friend Mr. Nelson possesses a slide of 

 Navicula rhomhoides in quinidine which has been photographed 

 for Carpenter's work on the microscope, and which for ten years 

 at least has shown little sign of change. But, generally speaking, 

 quinidine mounts become opaque in the course of a few days, from 

 the crystallisation of the medium ; and then the only alternative 

 is once more to fuse the crystals, which often results in the dis- 

 placement of perchance your favourite valve, and consequently 

 the ruin of your slide. 



Realgar has occasionally been used as a mounting medium for 

 diatoms. It possesses the high refractive index of 2*5, and con- 

 sequently is a valuable means of " forcing out," so to speak, the 

 ultimate structure of a suitable object. Its use, however, is 

 attended by several drawbacks. In the first place, fusion of 

 the material which is necessary for the mounting process requires 

 the application of great heat, which liberates intensely poisonous 

 fumes, thus constituting a real danger to life. In the second 

 place, the high temperature employed frequently twists or 

 distorts the valves, so that the}" are seldom found to lie flat 

 on the cooling of the shde. And lastly, the colour of the finished 

 mount is a deep yellow, and this seriously detracts from its value 

 for purposes of critical examination. This last defect, however, 

 admits of at least partial remedy by the use of suitably tinted 

 glass screens interposed between the source of light . and the 

 condenser. In place of the malachite-green screen I have been 

 in the habit of using a polished plate of bright blue glass, and 

 this with the yellow medium forms a suitable combination, which 

 I have found very pleasant to the eye, as well as helpful in 

 aiding the resolution of diflicult tests. 



Whilst examining one of the earlier and less perfect mounts in 

 realgar that found their way here from Germany, and which 

 was so full of " air bubbles," or colourless spaces that it was 

 discarded as unfit for sale by one of our principal opticians, I 

 was surprised to find that a very perfect valve of Amj^hipleiira 

 2)elhicida, which lay, evidently b}^ chance, in one of these 

 colourless spaces, was resolved as easily as were an}" of the 

 other valves lying in the deep yellow realgar ! ^Yhat is the 



