1892.] MICROSCOPICAL JOURNAL. 229 



diamond. A large glass cover is cut in half and a piece cemented 

 to the slip on each side of the mark, leaving a space of about 

 ^ inch between their edges. A vei'y thin cover-glass is then 

 placed upon them, thus forming a cover to a rectangular cell, in 

 which the liquid to be examined is placed. The fine mark is 

 now viewed with the highest power available, and when the 

 focus is sharp the position of the fine adjustment is noted. The 

 slip is then removed, another fluid substituted, using the same 

 cover-glass, and the operation is repeated. The change in the 

 focal adjustment required is a measure of the diflerence in the 

 refractive indices of the two liquids. In this way the relative 

 refractive powers of two or more media can be immediately de- 

 termined. 



To measure the absolute index of refraction of a fluid it is nec- 

 essary to obtain a numerical value of the graduations of the fine 

 adjustment. To do this, choose two liquids of known refractive 

 power, one of low index, as water, and the other of high, as oil 

 of cassia. Focus through them both successively, and note the 

 alteration of focus required. The indices of the fluids being 

 known, a value for each division of the fine adjustment is easily 

 obtained by calculation. 



Another simple and quite inexpensive device for testing the 

 refractive index has been described by Prof. H. L. Smith. The 

 necessary apparatus consists of an adapter about J-inch long, 

 with society screw outside and inside. A horizontal slot is cut 

 into each side of the adapter ; through these slots are made to 

 glide two slips of crown-glass (2 in. x ^ in.), having very ap- 

 proximately the refractive index of ordinary cover-glass. In one 

 of these slips, near the end, a concave is ground, at least ^ the 

 thickness of the glass, and polished. The adapter, with slips, is 

 attached to a microscope and carries a i-inch objective. To 

 graduate the instrument for use as a refrac to meter, make a mark 

 on the rack-bar when the focus is perfect for an object, viewed 

 with the I -inch objective, and the slides so arranged that the end 

 without concave is within the adapter ; this mark represents a 

 refraction index of 1.52 ; now fill the concave with oil of cassia, 

 push the slip into the adapter so that the concave is within ; 

 focus the object carefully and make another mark on the rack- 

 bar ; this represents a refractive index of i.6-\-; by doing the 

 the same with water you get the point which represents a refrac- 

 tive index of 1.33; glycerine gives the position for a refractive 

 index of 1.41. The extremes may be one-half inch apart. By 

 interpolating, quite a complete graduation can be obtained, suffi- 

 cient, at any rate, for the ordinary use of the microscopist. 



This little apparatus was particularly devised to test homogene- 

 ous immersion media and has been called Professor Smith's Homo- 

 tester. A good immersion fluid should give best focus at the 

 1.52 mark, and the image should be as free from color as 

 possible. 



