204 



USE OF THE MICROSCOPE. 



equivalent to a dark spot or disc placed 

 upon the convex face, and it suggested 

 to Mr. Thomas Ross the idea of employ- 

 ing a very thick or nearly spherical 

 lens, or a globe "of glass full of water, 

 termed a candle cracker, and stopping 

 out the central rays by means of discs 

 ^^ of tin foil blackened on one side, and 

 cemented to two opposite surfaces, 

 Mr. Topping has since carried out Mr. 

 Thomas Ross's plan and employed both 

 a candle cracker and an artificial white 

 eye of a bird for the same purpose, 

 both of which answer exceedingly well : 

 a stage fitted with one or more of these 

 illuminators, for object-glasses of different focal lengths, is 

 supplied by Mr. Topping. 



Amici's Illuminator. — This illuminator may either be adapted 

 beneath the stage of the microscope by means of a jointed arm, 

 or be mounted on a separate stand in the manner shown by 

 fig. 133, a plan adopted by M. Nachet, for which the author is 

 indebted to Mr. Warren De la Rue. The foot is of brass, of 



Fig. 132. 



Fig. 133. , 



the shape represented by A : with this are connected three thin 

 jointed rods, B, C, D, the illuminator, E, being readily turned 



