Binocular Microscopes. 35 



taneously, saving the strain which necessarily ensues 

 through the constant employment of the monocular 

 microscope, and the endeavour to see in the best manner 

 the detail in the specimens. We should recommend every 

 buyer of the monocular microscope to train himself to use 

 either eye, keeping the one not in use open ; this will be 

 found of the very greatest service. The universally under- 

 stood binocular microscope consists of a prism designed by 

 Wenham, which admits of the light going up a direct tube, 

 and reflects light also into a second tube. By this means 

 it is claimed that objects can be seen more naturally than 

 with the monocular microscope, for the reason that 

 stereoscopic vision is obtained, and objects having a certain 

 amount of depth may be seen completely with the binocular 

 microscope, whereas with the monocular it would be 

 necessary to focus through the entire depth. These effects 

 are very beautiful, but it must be borne in mind that this 

 form of binocular cannot be used with an objective having 

 a higher air angle than 40. Provision is, however, always 

 made whereby the prism may be withdrawn, and the 

 light then only goes up the monocular or straight tube, 

 and the instrument is to all intents and purposes as useful 

 and convenient as the monocular, while the observer can 

 rest his unused eye by looking down the blank binocular 

 tube, when the effect of its not being illuminated will 

 scarcely be noticed. It must be understood that all vision 

 through the microscope in the ordinary way is inverted, 

 that is, the object is seen upside down. A very good form 

 of binocular microscope, devised by Stevenson and made 

 by Swift and Son, erects the image, or, in other words, 

 it is seen the right way up. It is well spoken of for dis- 

 secting purposes, and high powers can be employed with 

 it ; still, it cannot be described as an all-round microscope, 

 and would have to be classed with instruments for special 

 work. 



For use with the binocular microscope, the closer the 

 posterior lens of the objective is brought to the prism the 



