THE AMERICAN 



MONTHLY 



MICROSCOPICAL JOURNAL. 



Vol. YI. 



Washington, D. C, October, 1885. 



No. 10. 



Device for Testing Refi'tictive 

 Index.* 



A new device for testing the re- 

 fractive index of immersion media, 

 and indicating how near an aj^proach 

 to homogeneity with crown-glass can 

 be made, was described, at the recent 

 meeting of the American Society of 

 Microscopists. by Prof. H. L. Smith, 

 who claims for this simple device 

 superiority, both as to ease of manip- 

 ulation and accuracy of indication, 

 over the well-known wedge and bot- 

 tle furnished by Mr. Zeiss. 



In testing any medium for immer- 

 sion purposes, but little more than a 

 drop of the liquid is required, and 

 the slightest variations of refractive 

 index are indicated by a considei'able 

 latitude of motion, when, in the or- 

 dinary use of the wedge, these varia- 

 tions would be inappreciable. The 

 instrument is used upon the micro- 

 scope, and a reference to fig. 37 will 



KiG. 27. — Device for Testing Rrfrautive Index. 



make the application plain. A is an 

 adapter about three-fourths of an inch 

 in length, with the societv screw out- 

 side and inside. This is attached to 

 the microscope, and carries a one-inch 

 objective, a and 6 are two slips of 

 crown-glass, as near the refractive 

 index of the cover-glass as possible, 

 two inches long and half an inch wide, 

 each about a tenth of an inch in thick- 



* Revised by the author. 



ness. In one of these, 6, near the 

 end, a concave is ground to a depth 

 of about one-third or more of the 

 thickness of the glass, and polished. 

 To test whether a medium has the 

 same refractive index as the glass, 

 and also the dispersion, a drop of it 

 is put into the concave, and the two 

 slips of glass are placed together and 

 inserted into an opening 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 one-inch ob- 

 jective is screwed on below, and the 

 microscope is focussed on some well- 

 defined object on the stage. Look- 

 ing through the two slips in this way, 

 the focus will be found not to difler 

 .appreciably from what it would be if 

 the glass plates were removed. When 

 the object is clearl}' defined the plates 

 are pushed in, bringing the concave, 

 filled with the liquid, direcdy over 

 the back of the objective ; if the me- 

 diuin 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 un- 

 changed, but an excess of color will 

 fringe the outlines of the object. If 

 the focus has been obtained by means 

 of the rack and pinion, the fine ad- 

 justment always remaining the same, 

 one can readily ascertain the refrac- 



