ILLUSTKATOES. 



205 



on the last one, D, by means of the lever, F. The illuminator 

 itself consists of a portion of a large thick lens, such as that 

 represented by G, as being cut off from H. In order to 

 explain its action the course of three rays of light, ah c, is 

 represented : these rays, on reaching the first surface, are ren- 

 dered slightly convergent ; on meeting with the plane surface 

 they are reflected at right angles, as indicated by a' V d; and on 

 passing through the convex surface G they are converged into 

 a focus at I. When in use the illuminator is placed imme- 

 diately below the stage, and the light incident on one convex 

 surface is converged on the object by the other, and by 

 turning, the handle, F, various degrees of obliquity may be 

 obtained; it may also be employed as a substitute for the 

 buU's eye condenser to iQuminate opaque objects on the stage : 

 in this case the position of the reflecting surfaces must be the 

 reverse of that indicated in the figure — in other words, the 

 plane surface must be uppermost. 



Annular Condenser. — In order to take advantage of the 

 reflection from the internal surface of glass Mr. Shadbolt has 

 contrived the instrument represented by fig. 134, which he 

 terms the annular condenser ; it consists of an annular prism 



Fig. 134. 



or ring of glass, which, if divided vertically, would be repre^ 

 sented by a in No. 1, fig. 134, the two upper planes being 

 inclined towards each other according to the obliquity of the 



