168 THE MICROSCOPE. 



of the reflecting surfaces from coming into operation, 

 which can seldom be made very perfect. 



" The definition with these prisms is good ; but they 

 are liable to objection on account of the extremely small 

 portion of the field of view that they take in, and which 

 arises from the distance that the eyes are of necessity 

 placed beyond the focus of the eye-piece, where, the rays 

 being divergent, the pupil of the eye is incapable of taking 

 them all in ; also there is great nicety required in the 

 length of the prisms, which must differ for nearly every 

 different observer. 



" I have constructed an adjusting binocular eye-piece, 

 not differing in principle from the last. The first reflection 

 is performed by means of a triangular steel prism, with 

 the two inclined facets very highly polished; this is repre- 

 sented by the dotted outline g g. The rays, after having 

 been reflected at right angles, are taken up by two rec- 

 tangular glass prisms, shown by the dotted lines at //. 

 The best effect that I have yet produced in the way of 

 binocular vision applied to the microscope, is that next to 

 be described, in which I have altogether dispensed with 

 reflecting surfaces, merely using three refracting prisms, 

 which, when placed together, are perfectly achromatic. 

 a a, diagram 2, fig. 97, is a single prism of dense flint- 

 glass, with the three surfaces well polished ; b b are two 

 prisms- of crown-glass of half the length of the under flint- 

 prism, to the upper inclines of which they are cemented 

 with Canada balsam. 



" The angle of inclination to be given to the prisms 

 must depend upon the dispersive power of the flint and 

 crown glas& employed. In the combination that I have 

 worked out, I have used, for the sake of simplicity, some 

 flint and crown that Mr. Smith kindly furnished me with, 

 in which the dispersive powers are exactly as two to one ; 

 consequently, I have had to make the angle of the crown 

 just double that of the flint, in order to obtain perfect 

 achromatism. The refractive power of each must also be 

 known, that we may determine the angles of the prisms 

 suitable for refracting the rays from the object-glass into 

 the two eyes, at a distance of nine inches, c, fig. 97, 

 represents a ray of light incident at right angles upon the 



