Immersion Lenses and New Befradometers. 77 



experiments made with the Nobert test-plate, and from the most 

 careful examination with the best English and American objectives, 

 the most experienced American microscopists believe there is some 

 mistake." This object has been so frequently seen in this country 

 that English microscopists would be glad to hear whether our Ame- 

 rican friends still disbelieve in its resolution, as this fact would be 

 a strong argument against this supposed " American superiority." 



It is a highly curious psychological phenomenon, that whenever 

 some new advanced microscopical facts are discovered, old-fashioned 

 observers honestly believe their advocates to be the victims of 

 delusion, spurious appearances, ghosts, or mistakes. 



IV. Description of two Refractometers, — the 07ie Mechanical, 

 the other Logarith^nic. 



Experiencing the tedious work of constructing tables of de- 

 viation for rays of light passing through successive media of known 

 refractive power, it occurred to me that an instrument could be 

 contrived which should mechanically give the angles of refraction 

 for any angle of incidence between any known media. 



Let a triangle be constructed none of whose angles are greater 

 than 90°, it is well known that the sides are in the exact proportion 

 of the sines of the opposite angles. Thus if A B C be the triangle, 

 a, h, c, the sides in inches, 



a : 6 : : sin. A : sin. B. 



Now, suppose that a, b, represent the refractive indices of two 

 substances, then the sines of the angles of refraction and incidence 



are as a and h. From this it follows, that if a triangle be con- 

 structed whose sides represent the refractive indices of two given 



G 2 



