ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



763 



apparatus in fig. 108, and the hollow space contains two lenses cemented 

 to the spherical surfaces as shown. The condenser gives ordinary light- 

 ground effects when K is swung out, and dark-ground when it is 

 replaced. 



The author emphasizes H. Siedentopf's observation that apochromatic 

 immersion systems give much better dark-ground results than are 

 obtained with apochromatic dry systems. 



Glass and Metallic Replicas of Gratings.* — J. A. Anderson points 

 out that replicas of gratings were first f made by Thorpe in England, 

 and later, by Wallace and by Ives in America. The method used by 

 Wallace and Ives is to pour upon the grating a solution of gun-cotton 

 in amyl acetate, or some similar substance, and after this is dry to allow 

 it to peel off under water, and then to mount it upon a piece of plane 

 glass. One surface of the film of collodion, the one which was in 

 immediate contact with the surface of the grating, is found to be a fairly 

 accurate copy of the ruled surface of the grating itself, while the other 

 one is more or less perfectly flat. The first will be spoken of simply as 



yw 



Collodion 

 Film 



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._ 6ia 55 



Fig. 110. 



Pig. 111. 



the ruled surface, or as the face. Thorpe mounted his replicas with the 

 ruled surface up, while Wallace speaks of mounting the film either ruled 

 surface up or down, preference being given to the latter. It is believed 

 that Ives mounts all of his ruled surface down in contact with the glass. 

 When a replica is mounted face up, it may he transformed into a 

 metallic reflection grating by simply coating it with platinum, by means 

 of cathode disintegration in a vacuum, as has been lately described by 

 E. Gehrke and C. Leithauser. As a rule, however, this surface is, 

 perhaps, never quite plane owing to the unavoidable differences in the 

 thickness of the film in different places. The author thinks that 

 gratings made in this way will never perform very well when subject 

 to a really severe test. The author, being fortunate enough to have at 

 his disposal a very large number of Rowland gratings, has endeavoured 

 to ascertain " How perfect is it possible to make a replica ? " — or, what 

 amounts to the same thing, " How nearly will the resolving power of a 

 replica equal that of the grating from which it was made ? " It was 

 found that difficulties arose from the drying of the film, as it had a 

 tendency to shrink. This difficulty increased with the size of the 

 grating. The author found, however, that by mechanically stretching 

 the replica wherever it required it, he was able with practice to correct 



* Johns Hopkins Univ. Circular, No. 2 (1910) pp. 19-23 (2 figs.). 

 t Replicas of gratings in collodion were first made more than half a century ago 

 by the Rev. W. Hodgson.— rED. Journ. R.M.S.] 



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