176 S. HOLMES ON A NEW FORM OF BINOCULAR MICROSCOPE. 



parent magnitude of the two images, and finally projecting them 

 into the respective body tubes of the instrument. But as in 

 the concrete form, both lenses and reflectors are entire, they have 

 but two common surfaces, instead of six, and will not there- 

 fore introduce any indistinctness by secondary reflection, for the 

 rays traverse the same solid medium until their final emergence, 

 and further the angle of inclination is such as to act by total 

 reflection and thus the illumination of the image is not weakened 

 by the change of direction. 



The smallest amount of spherical aberration attends this position 

 of a thick piano convex lens, and the evils arising from bad centring 

 cannot exist, for the surfaces of both lenses are ground in the same 

 tool, and are in fact, but contiguous prolongations of the same surface. 

 This angular position of a hemispherical plano-convex lens was 

 first proposed by the late Sir David Brewster, only for doubling its 

 magnifying power, and for use as a diagonal eye-piece, hut hy the 

 addition of an angulated surface the duplication of the image is 

 secured for projection through binocular eye-pieces. 



If the angulated surface is cut into the lens, the resulting image 

 should be pseudoscopical ; but if the surfaces are made by cutting 

 off the lens, the right eye will receive the left-hand image and 

 vice versa, and the result should be a stereoscopic representation of 

 any opaque object under notice. 



I am sorry to say that I cannot exhibit to the meeting a com- 

 plete instrument on the plan suggested, but I must content myself 

 with offering for inspection the lens I have exj)erimented with. 



It would not be difficult to render such a lens achromatic, by a 

 flint concave, and any magnifying power may be had by screwing 

 on, underneath the reflecting lens, the ordinary objectives. 



This lens, it will ' be observed, I have made of two pieces, and 

 cemented them together with Canada balsam, because difficulties 

 of construction leave no means to make it of one piece. Perhaps 

 it should have been joined by some black opaque cement, to ensure 

 complete isolation of each semi-field of light. 



12, Brunswick Terrace, Lower Eoad, Botherhithe. 

 November %th, 1868. 



