MICROSCOPE The first binocular microscope was invented by the previously mentioned Father Cherubin, whose instrument consisted of two inverting systems and consequently gave a totally wrong impres sion of depth, i.e., the depressions appeared as elevations and vice versa or, according to Charles Wheatstone's phrase, it presented a pseudoscopic impression. This quality was not recognized by the microscopists of the time. The instrument subsequently fell into complete neglect for nearly two centuries, to be revived in 1852 by Charles Wheatstone. The publication of his views in his second great paper on "Binocular Vision" (Phil. Trans. 1852) undoubtedly stimulated the investigation of this instrument, which was carried on with zeal and success more especially in England and the United States of America. In 18J3 the American J. L. Riddell published a description of his binocular microscope which contained the essentials of Wheatstone's pseudoscope. Wheat stone tried without success to interest Andrew Ross and Hugh Powell, the two most important microscope makers in London, in the binocular microscope.
Some time later, however, F. H. Wenham became interested. He was not only a gifted amateur but also a technician having a particular interest in the advancement of the microscopes, and his theoretical insight in the subject was remarkable. His en deavours however bore fruit only in the next decade, but in a way which was decisive for the adop tion of the binocular microscope in England at any rate. His first construction was almost identical with that of J. L. Riddell (fig. 12), but this type he only con sidered useful for binocular mag nifying glasses. He tried to use Riddell's arrangement between the objectives and eyepieces but he could not avoid the pseudoscopic effect. The same defect adhered to his first achromatic refracting prism (fig. 13) inter posed between the eyepieces and objectives of the microscope.
In the spring of 186o he brought out his first binocular micro scope (fig. 14) with orthoscopic representation. He arranged the achromatic refracting prism so that the emergent rays crossed over on their way to the eyepieces so that the rays from the left side of the objective entered the right eyepiece and vice versa.
This binocular microscope had the disadvantage of not being able to be used as a monocular microscope, and Wenham published in Dec. 186o a description of his reflecting prism (fig. 15). When placed close behind the objective it directs the rays coming from the right half of the objective after two reflections into the left eyepiece of the microscope ; those coming from the left half of the objective go straight into the right eye piece (fig. 16). This form of binocular microscope construction was eminently suited to the English tube length of 1 o in., but for continental microscopes with their shorter tube length (6 2 in.) the required deviation in the reflecting prism became considerable and other constructions have been adopted in this case. Of other work ers in this field mention may be made of Alfred Nachet, who in 1853 and subse quently in 1863 brought forward two forms of binocular microscopes.
The earlier stages of the development of the binocular microscope had always been confined to those instruments with one objective in the immediate neighbourhood of which the systems for dividing the pen cils were placed. At a later date, and par ticularly on the continent and in America, attempts were made to separate the image produced by the objective by modifying the eyepiece; this led to the construction of stereoscopic eyepieces introduced by H. B. Tolles, A. Prawsmosi and Abbe. The eyepiece designed by E. Abbe is of special impor tance, although, as he has stated, his methods accidentally led him to a construction given by F. H. Wen ham in 1866. Wenham was then endeavour ing to construct binocular microscopes for binocular vision as distinct from stereo scopic vision. He was of opinion that with objectives of a focal length of 5 mm. or less the image quality suffered consider ably through the introduction of the stereo scopic arrangements, but that for such short foci the binocular vision was still of advan tage although it offered no appreciation of depth. Wenham's prism form is shown in fig. 17. He split each beam from the objective into two portions, one going straight through the prism block, and the other being reflected on a prism surface bounded by a thin air-space, and then totally reflected on the further side of the prism to be directed into the other tube. E. Abbe's eyepiece construction fol lowed these lines closely (fig. 18) . The eye piece without special arrangements did not present a stereoscopic image of the object but afforded simply binocular vision ; by adapting special diaphragms over the eye pieces the image could be made to appear either orthoscopic or pseudoscopic, ac cording to whether the outer or the inner halves of the exit beams of the two eye pieces were allowed to enter the observer's eye. A fundamentally different type of binocular microscope (fig. 19) with ortho scopic representation of the image has been designed by H. S. Greenough by em ploying two separate tubes, each complete with objective and eyepiece, together with a Porro prism system to erect the images.
By making these Porro prisms rotate round the tube axis the interocular distance could be adjusted to suit the observer.