ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



1067 



cut in the adapter-tube, as shown in the figure. A thin stratum of 

 the medium will flow between the two slips. The whole being now 

 in the position shown in the figure, the 1 in. objective is screwed on 

 below, and the Microscope is focused on some well-defined object on 

 the stage. Looking through the two slips in this way, the focus will 

 be found not to differ appreciably from what it would be if the glass 

 plates were removed. When the object is clearly defined the plates 

 are pushed in, bringing the concave, filled with the liquid, directly over 

 the back of the objective ; if the medium be optically homogeneous 

 with the glass slips, there will be neither spherical nor chromatic 

 aberrations produced, and the definition and focus remain unchanged. 

 As none of the immersion media now known are strictly homogeneous 

 in this sense, but may, nevertheless, have the same mean refractive 

 index as the crown glass, clear vision with these will be obtained 

 with the general focus unchanged, but an excess of colour will fringe 

 the outlines of the object. 



If the focus has been obtained by means of the rack and pinion, 

 the fine adjustment always remaining the same, one can readily 

 ascertain the refractive indices of various media proposed for use 

 with immersion objectives in this way. Let a mark be made on the 

 rack-bar or sliding tube, as the case may be, when the focus is 

 obtained with the plates in the position shown in the figure ; this 

 mark will indicate, for example, a refractive index of 1 • 52. Filling 

 the concave now with cinnamon oil, and focusing again (using the 

 same object, objective, and eye-piece), we get another position for a 

 mark indicating a refractive index of 1 * 6. Using water, we get still 

 another, 1'33, and with glycerin 1 • 41, the extremes will be about 1/2 in. 

 apart, as measured on the bar or tube, and, by interpolating, we can 

 thus get pretty nearly the refractive index of any fluid medium. 

 Prof. Smith has found the so-called homogeneous media sold in the 

 shops to differ very greatly, fully 1/4 in. out of the way in many cases. 

 A specimen of cedar oil from Zeiss caused a change of focus only 

 about 1/20 in., which was less than was required by any other 

 samples 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 certain tests, the exact refractive index of the medium can 

 be ascertained, and afterwards always secured. A non-adjustable 

 immersion objective, an 1/8 by Spencer, which performed most ad- 

 mirably, both with oblique and direct light with the medium furnished 

 by the maker, showed but indifferently 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 1/4 in. 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 

 prevent 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 through the fluid. 



