54 SIMPLE MICROSCOPES. 



the smaller plate. The left-hand back pillar contains a triangular 

 bar -with rack-and-pinion movement, for focal adjustment, which 

 carries the horizontal arm for the support of the magnifiers ; this 



Fig. 30. 



Beck's Dissecting Microscope, with Nachet's Binocular Magnifier. 



arm can be turned away towards the left side, but it is provided 

 with a stop which checks it in the opposite direction, when the 

 Magnifier is exactly over the centre of the Stage-aperture. Beneath 

 this aperture is a concave Mirror, which, when not in use, lies in 

 a recess in the mahogany base, so as to leave the space beneath the 

 stage entirely free to receive a box containing apparatus ; whilst 

 from the right-hand back corner there can be raised a stem carrying 

 a side Condensing-lens, with a ball-and-socket movement. In addi- 

 tion to the single Lenses and Coddington ordinarily used for the 

 purposes of dissection, a Binocular arrangement was devised by 

 Mr. It. Beck, * on the principle applied by MM. Nachet, about the 

 same date, in their Stereo-pseudoscopic Microscope (§ 29). For, 

 adopting Mr. Wenham's method of allowing half the cone of rays 

 to proceed to one eye without interruption, he caused the other 

 half to be intercepted by a pair of Prisms disposed as in Fig. 22, 2, 

 and to be by them transmitted to the other eye. It will be readily 



* " Transactions of the Microscopical Society," N. S. Vol. xii. p. 3. 





