ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 



653 



the vertical 60° backwards — that is to say the prisms a a are turned 

 1 round on their bases in the diagram to show the direction of the 

 rays. In future constructions, Mr. Ahrens thinks the prisms a and h 

 should be cemented together. 



Fig. 136. 



We fear that this Microscope cannot compare with the simplicity 

 of the Wenham and Stephenson models, or even with those which 

 have been previously devised by Mr. Ahrens himself. 



Crossley's Microscope with Special Arrangement for lUumi- 

 nating the Swinging Substage. — This arrangement is the design of 

 Mr. Edward Crossley, F.E.A.S. A general view of the Microscope, 

 as constructed by Messrs. Watson, is given in Fig. 137, and Fig. 138 

 shows in section the prisms used for the illumination. 



The light from the lamp is thrown into the hollow horizontal 

 axis of the Microscope with the aid of the bull's-eye condenser, and 

 by a prism placed in the centre of this axis, is reflected forwards 

 in the direction of the axis on which the swinging substage turns. 

 The arm of the swinging substage is made in the form of a box, and 

 carries a second prism in the axis on which it moves, so as to intercept 

 the rays of light coming from the first prism and reflect them in the 

 direction of the arm or box. At the end of the box is a third prism, 

 which throws the rays of light forward on to the mirror, by means of 

 which they are finally directed to the object on the stage. 



In Fig. 188, A is the first prism, as seen in vertical section in the 

 centre of the horizontal axis. B is the second prism in the axis of 

 the swinging substage. C is the third prism at the end of the box 

 arm of the swinging substage. D is the mirror, and E is the stage 

 carrying the object. The dotted line represents the direction taken 

 by the rays from the lamp. 



It will thus be seen that no change in position of the Microscope 

 on its horizontal axis affects the direction of the light from the lamp, 

 and also that whatever the position of the swinging substage, whether 

 above or below the stage, the illumination remains constant upon 



Ser. 2.— Vol. I. 2 S 



