these visual pictures brought into coincidence, the same conception of a solid form is generated in the mind just as if the object itself were there. Professor Abbe, however, contended that the method by which dissimilar images are formed in the binocular microscope differs materially from that of ordinary stereoscopic vision, and that the pictures are united solely by the activity of the brain, not by the prisms which ordinarily give rise to sensations of _solidity_. This can be only partially true, as binocularity in the microscope is due to difference of projection exhibited by the different parallax displacement of the images, and also to the perception of depth imparted by the instrument. Wheatstone was firmly convinced that his stereoscopic principle could be applied to the microscope, and he therefore applied first to Ross and then to Powell to assist him in its adaptation. But whether either of these opticians made any attempt to give effect to his wishes and suggestions is not known. In the year 1851 Professor Riddell, of America, succeeded in constructing a binocular microscope by employing two rectangular prisms behind the objective. M. Nachet also constructed a binocular with two body-tubes and a series of prisms. But neither Riddell’s nor Nachet’s instrument was ever brought into use; they were either too complicated or too costly. It will be understood, however, that the binocular stereoscope combines two dissimilar pictures, while the binocular microscope simply enables the observer to look with both eyes at images which are essentially identical. Stereoscopic vision, to be effective, requires that the delineating pencil shall be equally separated, so that one portion of the admitted cone of light is conducted to one eye, and the other portion to the other eye. Select any object lying in an inclined position, and place it in the centre of the field of view of the microscope; then, with a card held close to the object-glass, stop off alternately the right or left hand portion of the front lens: it will then appear that during each alternate change certain parts of the object will change their relative positions. [Illustration: Fig. 40.--Portions of Eggs of Cimex.]