184 THE AMERICAN MONTHLY [August, 



a convex action, requiring for neutralization a concave lens of .25 

 D. I, therefore, from experiment with my device do not hesitate 

 to say that this specimen of oil of cinnamon was adulterated with, 

 probably, oil of cassia, whose refractive index is seen in the table 

 to be much higher. 



With oil of fennel and oil of cinnamon thei'e was an evident 

 prismatic effect, color dispersion being well observed at the edge 

 of the concave. With oil of cloves this was much more marked, 

 and with oil of cassia the prismatic colors were so intense as to 

 impair the view of the window sash. 



With homogeneous immersion fluids or fluids sold as such, I 

 had equally interesting and confirmatory results. The first one 

 already alluded to gave, as has been stated, negative refraction re- 

 quiring a convex lens of i. D. for neutralization. It is better than 

 commercial cedar oil, but I fear its index is too low for effecting 

 the best results with a homogeneous immersion lens. 



Messrs. J. W. Qiieen & Co.'s homogeneous immersion fluid 

 absolutely neutralized all refractive influence of the concave when 

 viewing a window frame 6 or 8 feet away. When viewing an 

 object 100 or 150 feet distant there was evident on close observa- 

 tion a slight degree of convex lens action, requiring for neutrali- 

 zation a concave lens of .12 D. As the focal length of this lens 

 is somewhere about 2SS inches, or 24 feet, it will be seen that the 

 variation of the index of this fluid from that of the glass is practi- 

 cally «//, and moreover the variation is a convex one, which is an 

 advantage. 



Messrs. Bausch & Lomb's hom. imm. fluid is also practically 

 homogeneous. Viewing through the device filled with it a win- 

 dow frame 6 or 8 feet distant, the faintest apparent motion in op- 

 posite direction (that is, a convex-lens action) could be made out, 

 while the apparent opposite motion of an object thus viewed at a 

 distance of 100 or 150 feet, while evident, was slight, requiring 

 a concave lens of .25 D. for complete neutralization. This fluid 

 thus is practically homogeneous and the variation is in the right 

 direction. 



A specimen of thickened cedar oil sent out for use with hom. 

 imm. objectives by a celebrated English firm of microscope mak- 

 ers had a refractive index less than that of the glass in my device, 

 requiring for neutralization a convex lens of greater strength than 

 .25 D., a .5 D. being too strong; we might take the mean and 

 estimate the residual concave action in using this fluid as 37 D. 

 The variation is in the wrong direction. 



It must be borne in mind that the foregoing experiments and 

 the deductions therefrom, excepting always the loss of parallax 

 if the fluid is homogeneous, are based upon the individual slips 

 of glass and the negative focus of concave actually used. An ex- 

 tension of the number of fluids thus tested would enable us to 

 gather data from which might be constructed a mathematical 

 curve from which we could determine, l)y the method of interpo- 



