145 



On a Neutral Tint " Selenite Stage." 

 By W. Ackland. 



{Read November 25th, 1870.) 



la using polarized light with the microscope, many objects 

 possess so slight a depolarizing influence, that a selenite film must 

 be used to bring out some structure not otherwise visible, or a dis- 

 play of colours that could not otherwise be obtained. 



Selenite films, yielding the various tints of blue, green, yellow, 

 red, and purple, are commonly employed ; but, anyone using these, 

 must have noticed, when viewing an object not entirely filling up 

 the field of view, that the colour of the background thus formed 

 fails to harmonize with the colours of the object under examination ; 

 and, indeed, more frequently the effect is considerably marred by 

 want of contrast. 



Now, my object in addressing you this evening is to recommend 

 you to try the neutral or pale bluish violet colour, corresponding to 

 the tint that occurs in Newton's rings, between the violet of the 

 second and the indigo of the third wave, and as used by Soleil in 

 his Saccharometer. 



This tint cannot be readily. obtained by splitting selenite, but is 

 easily produced by superimposing two suitable films ; and, when 

 thus obtained, is exceedingly delicate in action, as its colour is 

 varied by the slightest depolarizing influence of the object under 

 examination ; indeed, it is often changed to either violet or blue by 

 the action of the thin glass cover, or a slight tension of the mount- 

 ing medium. 



To gain the fullest advantage of this neutral tint, I have devised 

 an efficient, but simple selenite stage, which, when in use, is laid on 

 the microscope stage and the object on it. 



It consists of two films — one the primary, capable of being 

 rotated by the milled head on the right-hand side, and the second, 



