182 



THE AMERICAN MONTHLY 



[October, 



tive indices of various media pro- 

 posed for use with immersion objec- 

 tives in this w^ay. Let a mark be 

 made on the rack-bar or sliding tube, 

 as the case may be, wlien the focus is 

 obtained with the plates in the posi- 

 tion shown in the figure ; this inark 

 will indicate, for example, a refrac- 

 tive index of 1.53. Filling the con- 

 cave now with cinnamon oil, and 

 focussing again (using the same ob- 

 ject, objective, and eye-piece), we 

 get another position for a mark indi- 

 cating refractive index of i .6. LTsing 

 water, we get still another, 1.33, 

 and ■with glycerin 1.41. the extremes 

 will be about half an inch apart, as 

 measured on the bar or tube, and, by 

 interpolating, we can thus get pretty 

 nearly the refractive index of any 

 fluid medium. I have found the so- 

 called homogeneous media sold in 

 the shops to differ very greatly, fully 

 one-fourth of an inch out of the way 

 in many cases. A specimen of cedar 

 oil from Zeiss caused a change of 

 focus only about one-twentieth of an 

 inch, which was less than was re- 

 quired by any other samples I have 

 tried. 



When one has a fine objective, and 

 with a given immersion medium 

 has obtained certain positions of the 

 screw-collar for the best work on cer- 

 tain tests, the exact refractive index 

 of the medium can be ascertained, 

 and afterwards always secured. A 

 non-adjustable immersion objective, 

 a ^ by Spencer, which performed 

 most admii^ably, both with oblique 

 and direct light with the medium fur- 

 nished by the maker, showed but in- 

 diflerently well with another medium, 

 which, on being tested with the little 

 apparatus above described, required 

 an alteration of focus necessary to 

 obtain distinct vision, or rather the 

 most distinct vision, of fully one- 

 fourth of an inch. On diluting the 

 second medium to bring it to the 

 same index as that sent out by the 

 maker, the performance was entirely 

 satisfactory. It will be understood 

 that there should be a diaphragm in 



the adapter of such size as will pre- 

 vent any light passing through when 

 the concave is put over the objective 

 with the immersion fluid to be tested 

 in it, except what actually passes 

 througfh the fluid. 



New Cement and New Mounting 

 Medium. 



Prof. Hamilton Smith has com- 

 municated to the Editor the results 

 of some later experiments he has 

 made with a new cement, especially 

 adapted to protecting movmts in his 

 new stannous chloride mounting me- 

 dium, described in the September 

 Journal. It is made by diluting a 

 somewhat thick shellac cement, with 

 benzole, and adding sufficient litharge 

 to give a consistencv about the same 

 as that of white zinc cement. It 

 dries very quickly, forms a much 

 harder ring than does the white zinc 

 cement, and is not unpleasant in 

 appearance, as it becomes quite 

 brown, or dark on exposure. A 

 thin coat should first be applied, and 

 when this is well dried it should be 

 followed by another. So far as tried 

 this cement seems to promise better 

 than any other for preservation of the 

 stannous chloride mounts. The white 

 zinc often fails, and while the wax 

 rings appear to answer admirablv. 

 the cement is more readily applied, 

 and if the future use of it confirms 

 the present promise, it will be more 

 acceptable. 



In regard to the medium itself, the 

 refractive index may be raised con- 

 siderably by making a saturated solu- 

 tion in the glycerin jelly — about 60 

 grammes to the fluid dram — and 

 mixing this with the normal solution 

 of 40 grammes. By a saturated solu- 

 tion is meant one which, when thor- 

 oughl}' cooled, will show signs of 

 crystallization. The refractive index 

 in this case becomes nearly 2. 



Prof. Smitii writes that he is now 

 testing still another medium, of some- 

 what higher index than the stannous 

 chloride, a full account of which will 

 appear in due time. 



