INVEETEBRATA, CRYPTOGAMIA, MICROSCOPY, ETC. 



527 



direction of the ray R, and will bo totally reflected at the internal 

 surface S, whence they will fall upon the spherical surface of the 

 lens A, and be refracted nearly to its focus F. 



Spencer and ToUes Camera Lucida. — Mr. Stodder, of Boston 

 (U.S.), sends us a description of a camera lucida, which ho says is 

 largely used in America, and although very old has not hitherto been 

 described. 



The original form was the design of Spencer, and its construction 

 is shown in the diagram Fig. 46, where A A is the axis of the 

 Microscope, B the lens of the eye-piece, and C a prism, movmted in 

 front of B. The ray D from the pencil and paper passes through the 

 prism and reaches the eye at E, the ray from the eye-piece being 

 reflected at the upper surface of the prism, and also reaching the eye 

 at E. 



Fig. 46. 



Fig. 47. 



)!£>. 



I'.l. 



Fig. 48. 



The principle of this apparatus is obviously identical with that 

 of Dr. Bcale's " Neutral Tint lieflcctor," but it is claimed to be an 

 advantage that whilst the thin phito is liablo to give reflections from 

 both surfaces, the second surface in the Si)oucer form is so di.staut 

 that the internal reflection does not reach the eye. The head does 

 not require t<j be held immovable, but one may leave ofl' work and 

 begin again as often as desin^d. It can, moreover, be used witli hi"li 

 as well as low power objoctivcs. 



Mr, Tolles Kubsu(iueutly devi.sed a modiflcation of this camera. 



