1892.] MICKOSCOPICAL JOURNAL. 183 



through them an object several feet cHstant is observed there will 

 be no apparent lateral motion of such object when the combina- 

 tion is moved, if the refractive powers of the two lenses neutralize 

 each other. 



It will thus be seen that when the concave in one of the crown- 

 glass slips is filled to the plane surface (and to keep the fluid at 

 such plane is the only function of the other, plane, slip) with a 

 liquid of the same retractive index as that of the glass used there 

 is no refractive power left in the apparatus but it acts as tv/o 

 pieces of plane glass. If the fluid used have a refractive index 

 lower than that of the glass the combination is still of negative or 

 concave quality, and when the fluid is of higher refractive index 

 than that of the glass the mass of fluid in the concave has become 

 a convex lens whose focal length decreases, that is, the strength 

 of the lens is greater, in proportion as the refractive index of the 

 fluid is higher than that of the glass. 



The following experiments were made and many of them re- 

 peated with the same results. Concave equals approximately 

 concave lens of y^T^ inch negative focus. 



Concave filled with distilled water ; refraction negative and re- 

 quires a convex lens of 10.62 D. for neutralization. The refrac- 

 tive index of water is given as 1.333. The combination equalled 

 a negative lens of 10.62 D. 



With absolute alcohol (old specimen) = 

 '' glycerine " " = 



^ olive oil, good commercial (old 



specimen) = 



" oil of cedar" " 



from a first-class druggist. = 

 " oil of cedar from another druggist = — 3.35 D 

 "'• oil of cinnamon from first-class 



house. = 4- 



" oil of fennel '• " '^ = + 



" oil of cloves " " " = + 



" oil of cassia " "■ " = -j- 



In the foregoing it will be seen that oil of cedar, obtained from 

 a first-class house, when in the concave did not extinguish the 

 concave refraction, but left a residual amount that needed for com- 

 plete neutralization a positive lens of 1.5 D. ; that is, of about 30 

 inches focal length. This shows that even the best cedar oil of ' 

 commerce is not of sufliciently high refractive index for use as an 

 immersion fluid. The next in the list was evidently an impure 

 or adulterated specimen of cedar oil, as its refractive index is shown 

 to be less than that of an ordinary specimen of glycerine. 



Oil of cinnamon is given in tlie table in Carpenter, last edition, 

 as having a refractive index of 1.50S, or very nearly, but still less 

 than that of oil of cedar. It therefore ought to show in the con- 

 cave some i-esidue of concave action ; but the specimen used showed 



