124 THE MICHOSCOPE. 



ing the eye-lens h, if not intercepted, they would come to a focus at c ; 

 but they are arrested by the inclined surfaces, d d, of two solid glass 

 prisms. From the refraction of the under incident surface of the prisms, 

 the focus of the eye-piece becomes elongated, and falls within the sub- 

 stance of the glass at e. The rays then diverge, and after being re- 

 flected by the second inclined surface/ emerge from the upper side of 

 the prism, when their course is rendered still more divergent, as shown 

 by the figure. The reflecting angle that I have given to the prisms 

 is 47|-°. I also find it is requisite to grind away the contact edges 

 of the prisms, as represented, as it prevents the extreme margins 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 represented by the dotted outline g g. The rays, 

 after having been reflected at right angles, are taken up by two rect- 

 angular glass prisms, shown by the dotted lines a.tff. 



The loss of light in this is much greater than in the former instance, 

 and the field of view more contracted ; for the rays from the eye-piece, 

 after being reflected from the surface of the steel prism, fall to their 

 natural focal distance, instead of being elongated, as in the solid prism; 

 consequently the eye is still further removed from the focus. I had 

 chosen hard steel for the reflector, on account of the property this ma- 

 terial possesses of allowing the figure of a small flal surface to be re- 

 tained, or even perfected, during the operation of polishing. I have 

 also tried a combination of prisms over the field-glass, using two eye- 

 lenses ; but with no good result. The best effect that I have yet pro- 

 duced in the way of binocular vision applied to the microscope, is that 

 next to be described, in which I have altogether dispensed with reflect- 

 ing surfaces, merely using three refracting prisms, which, when placed 

 together, are perfectly achromatic, a a, diagram 2, fig. 89, is a single 

 prism of dense flint-glass, with the three surfaces well polished ; h b 

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



