The Microscope. 343 



added, 1 had a moment of anxiety, for the fluid became white and 

 opaque as milk, and I could see white particles falling on the lens 

 front, like little flakes of snow. Investigation proved that the salt 

 dissolved in the glycerine, a solution which makes so perfect an 

 immersion medium, acts chemically on the nickel plating of the 

 objective, and the glycerine, seizing the water, allowed the 

 new salt to fall in opaque white particles. The chemical action 

 is so great, that after using the medium but three times, there 

 was deposited on the cover of the test-plate an iridescent film, 

 having an irregularly circular outline, showing where the fluid 

 and the metal had been in contact. Nor is this all, for ac'oss 

 the surface of the front lens itself, is a streak of the same insol- 

 uble iridescent deposit. The optician, declines to make known 

 the composition of the fluid, although he might reveal it with 

 confidence, since no microscopisfc would ever make the medium 

 for his own use after having a little experience with it. Its 

 action on brass is similar to that on nickle, and must forbid its 

 use as an immersion medium, although it is really the equal of 

 the renowned cedar oil. To the latter, useful as it is, valid objec- 

 tions are its tendency to flow too freely, and the trouble needed 

 to clean it from the lens, alcohol being demanded to remove it 

 entirely, whereas with glycerine, a drop of water is enough. 

 Cannot some of our opticians give us a glycerine medium with 

 the refractive index of the resinated cedar oil, but without the 

 obnoxious quality of the fluid that acts on the objective mount- 

 ing? For these learned men, the problem should be an easy 

 one. The maker of the dangerous glycerine mixture can surely 

 make something as good ; I hope he will never make anything 

 quite so bad, although in its optical action it is as nearly perfect 

 as need be wished. Its hunger for metal is the fatal objection to it. 

 Upon the optical action of the immersion fluid depends the 

 optical action of the homogeneous immersion objective. If the 

 former is not of the proper index, the microscopist may deceive 

 himself by believing that his objectives are giving him the best 

 possible results ; or if they seem to be optically defective he should 

 remember that the fault maybe in the fluid supplied by the dealer. 

 The optician should place at the disposal of every microscopist 

 some simple device by which the refractive index of the immer- 

 sion medium may be ascertained. Zeiss sends out for this pur- 

 pose, a wedge of glass, which, when used as directed, gives the 



