MICROSCOPES. 213 



that the leading manufacturers do not know of their exist- 

 ence. Amongst these various instruments may be named 

 those constructed in the manner proposed by Nachet, 

 Wenham, Stephenson, Holmes, and Ahrens, but by far the 

 most usual are those made on the plan contrived by Mr. 

 Wenham. There is a small prism which can be pulled out or 

 pushed in, extending half over the object-glass. This prism 

 is so constructed that the light enters vertically, is 'twice 

 reflected, and passes out vertically, so that there is no dispersion, 

 and the beam remains colourless ; but by the second reflection 

 the light is sent up the oblique body of the microscope at such 

 an angle that the upper end of the two bodies are nearly at 

 the distance of our two eyes. The light which passes through 

 the other half of the object-glass not covered by the prism 

 passes up the direct body of the instrument. 



One great advantage in this system of Mr. Wenham is that 

 the light which passes straight up one of the tubes is not in 

 any way influenced by the prism; and the definition is there- 

 fore unimpaired. The definition of that half of the light which 

 passes through the prism is somewhat impaired, but this does 

 not signify very much, since the vision of the left eye is mainly 

 important in enabling us to distinguish differences in the 

 level of different parts of the object, and that of the right eye 

 gives good definition of the minute detail. Another great 

 advantage of this method is that you can at once convert the 

 instrument into a monocular microscope. In some cases it 

 is very desirable only to use one tube, and this you have a 

 means of doing by pulling out the prism. This is no doubt 

 the chief reason why this form of binocular has been so much 

 more commonly employed than any other, although I am by 

 no means certain that other forms are not to some extent 

 better, at all events when high powers are used. 



I have been informed that Mr. Stephenson's arrangement 

 gives exceedingly good results with very high powers, which 

 the other one will not. In it there are two truncated rectan- 

 gular prisms close behind the object-glass, so placed that they 

 reflect the light up the two bodies, both inclined to the direct 

 line of vision. By this reflection the object is inverted laterally, 

 and is still further inverted longitudinally by a larger prism, 

 so that finally the object is seen in its natural position, being 

 inverted by the object-glass and re-inverted by the prisms. 

 This instrument has the advantage of giving good results with 



