1892.] MICROSCOPICAL JOURNAL. 181 



A Simple Refractoiiieter. 



By JOSEPH T. O'CONNOR, M. D., 



NEW YORK CITY. 



In preparation for a summer to be spent in special investiga- 

 tions in biology, I have used all my spare time during the past 

 few w^eeks in wrestling with the management of a high-class i-io 

 horn. 1mm. objective from one of the best American makers. In 

 my hands the lens has not done what the makers claim it will 

 easily do and I naturally supposed that the fault lay in my own 

 want of manipulative skill. At the same time I was aware that 

 the immersion fluid (apparently a glycerine compound) furnished 

 by the maker and handed to me by his agent seemed darker than 

 it ought to be and felt that possibly it ought to bear a share of the 

 blame for my ill success. But how should I determine whether 

 or not the fluid was of the proper refractive quality.^ 



Reading, in the November number of Thp: Microscope, 

 Doctor Stokes' remarks crnccrning homogeneous immersion 

 fluids and his statement that Prof. H. L. Smith had devised 

 a simple and successful little refractometer but that it was 

 not in the market, I applied to a dealer and by good fortune ob- 

 tained one. The description of this instrument can be found in 

 the published proceedings of the Society of American Micros- 

 copists of 1SS5. It consists of an adapter that screws onto the 

 nose-piece of the microscope and receives at its lower end an 

 inch objective. Near the upper end of the adapter a slit is ar- 

 ranged (as for a micrometer eye-piece). Through this slit pass, 

 closely pressed together, two plates of crown glass of the same 

 refractive index as that of cover-glass. The glass plates are 3 

 inches long by ^ inch wide and about i-io inch thick. In the 

 middle of the flat surface of one plate is ground a " concave" 

 to ^ or more of the thickness of the glass. 



A suitable object, being now placed on the stage and lighted, 

 is focused as under ordinary circumstances. Next, two or three 

 drops of the immersion fluid are placed in the concavity, and the 

 other, plane glass slip is brought into apposition over the fluid 

 so that the cavity is completely filled ; a small portion runs out 

 to each side as an extremely thin layer, but this Prof. Smith finds 

 to be of no moment. The plates thus in apposition are passed 

 together through the slit in the adapter and it will be found that 

 when the plane surfaces only of the plates are in the pathway of 

 the rays from the objective no sensible change is needed in the 

 adjustment, but when the "concave" is brought into the field a 

 change in adjustment will be needed unless the fluid used was 

 really homogeneous, or, that is to sa}', of the same refractive index 

 as that of the crown glass. Prof. Smith found that by marking 

 the tube or rack-bar of the microscope a gauge could be made 

 showing the amount of adjustment needed for fluids of difl'erent 

 refractive indices, and thus, by interpolating, the refractive index 

 of an unknown fluid could be determined. 



