, 2*oe = 
27 
It occurred to him that a neutral tint, corresponding to the tint 
occurring in Newton’s rings, midway between the violet of the second 
wave and the indigo of the third wave, was the one required. In a parcel 
of 500 films he only found one giving this tint ; upon using it with an 
object he was delighted with the effect. Happening to put in its place a 
duplicate object, which, with an ordinary film, gave exactly the same 
colours, he was struck with its comparative poorness. Trying the first 
slide again, he obtained the same good result. This difference, hethought, 
was either owing to the thin glass cover being at the right axis for the 
colour or to the glycerine in which it was}mounted exerting an influence, 
As the neutral films were so difficult to obtain, he had tried a plan of 
rotating two films the one over the other, and thus obtained the 
neutral tint, so delicate in action that its colour varied by the slightest 
depolarizing influence of the object examined, and, at the same time, 
gave a display of colours more varied and gorgeous than could be 
obtained hy any of the usual films employed. At the same time, the 
rotation of one or other of the films gave a succession of nearly all the 
prismatic colours, so that, if dissatistied with the neutral tint, a multi- 
plicity of others could be produced. He had brought with him several 
selenite stages, which could be tested by the gentlemen present. 
Mr. Wonror, in moving a vote of thanks, said they were not only 
deeply indebted to Mr. Ackland for coming among them, but also for 
the valuable information he had afforded them. 
Mr. Sewetn seconded the resolution, which was carried 
unanimously. 
A very large number of polarizing objects of a varied character 
were then exhibited, with and without the selenite stages, by Messrs. 
Sewell, Turner, Ackland, Smith, Glaisyer, and Wonfor ; and general 
Satisfaction was experienced at iue results and the increased beauty 
with the neutral back- ground. 
Mr. Hennau also exhibited Dr. Piggott's ‘‘aplanatic searcher,” 
kindly lent by Mr. Curties, and Dr. Maddocks’ photo-casts of diatoms, 
ranging from 900 to 3,000 diameters, kindly lent by General 
Worcester. 
Mr. Wonror exhibited a searcher suggested by Mr. M'Intire, con- 
sisting of an objective arranged in the draw tube, with its observing 
end turned to the eye-piece, and a simple mode of mounting an 
