814 



Transactions of the Society. 



Fig. 152. 



My device (fig. 152) consists of a combination of a right-angled 

 prism (fig. 153), ABC, and a rliomboidal prism DEFGr, so arranged 

 that when adjusted very nearly in contact (i.e. separated by only a 

 thin stratum of au') the faces B C and D E are parallel, and conse- 

 quently between D E and B E' they act 

 together as a thick parallel plate of glass 

 through which the drawing-paper is viewed. 

 The rhomboidal prism is so constructed that 

 when the face G F is applied at right angles 

 to the optic axis of the Microscope the axial 

 ray H passes without refraction to I on the 

 internal face E F, whence it is totally re- 

 flected to J in the face D G. At J a part 

 of the ray is reflected to the eye by ordinary 

 reflection in the direction J K, and a part 

 transmitted to J' on the face A C of the right- 

 angled prism. Of the latter a portion is also 

 reflected to K by ordinary reflection at J'. 

 The hypothenuse face A C is cut at such an angle that the reflection 

 from J' coincides with that from J at the eye-point K, thus utilizing 

 the secondary reflection to strengthen the luminousness of the 

 image. The angle at G is arranged so that the extreme marginal 



Fig. 153. 



ray H' from the field of the B eye-piece strikes upon D G at a 

 point just beyond the angle of total reflection, the diffraction-bands 

 at the hmiting angle being faintly discernible at this edge of the 

 field. This angle gives the greatest amount of light by ordinary 

 reflection short of total reflection. 



By this arrangement the Eamsden circle over the eye-piece 

 comes just above the camera lucida, and the field of view is not in 

 any way reduced ; all that can be seen directly through the B eye- 

 piece (say 30° of field) is perfectly depicted in the camera lucida, 



