E. M. NELSON ON FINDING REFRACTIVE INDEX OF MOUNTING MEDIA. 9 



with the medium, and covering it with a piece of plane glass. 

 The formula for this latter plan being fx' =z fj, -[- ^ The radius 

 of the concave r might with advantage be made two inches, 



F = CO , / = I ; if F = 4, nx' = 2-0 ; and if 



