I 



. 116 



but let it be assumed that in applying the prism, the reflecting 

 suiface conforms to a deviation of 5 J as indicated by the dotted 

 line. A glance at the figure will 

 show that in this case (not 

 allowing for refraction in the 

 glass) the incident ray will he 

 reflected to RNi and the in- 

 cluded angle will be 100 - 

 instead of 90. 



Probably the first prism 

 to project an incident ray in- 

 variably at right angles with 

 its original direction was de- 

 signed by Dr. H. Wollaston, 

 an English scientist, in 1804 

 It is commonly known as 

 the"cameralucida" frequently 

 employed for perspective drawing in microscopical examination, but 

 rarely with the transit because of its limited field as compared with 

 its size and weight. 



The Wollaston Prism as shown in outline, Fig. 117, is the 

 quarter section of an octagon. The angle of reflection between AB 

 and BC - ERRi RRiF 135. If the beam of light can enter the 

 face anywhere between A and G, it will be reflected successively 



from AB and BC and ultimately 

 pass out between C and H so that 

 FN will be at right angles to the 

 incident beam ME, regardless of 

 slight displacements of the prism 

 in the plane of reflection. If a 

 perpendicular is dropped from B 

 upon the catheti at G and H, 

 there will be defined the limitation 

 of the sides AD and CD to which 

 B A and BC are related. The por- 

 tion of the prism block, GDH, is 

 YN therefore of no service and is dis- 



carded. If such a prism were to 



be employed for nadir sighting, or othersvise, the diameter of the 

 face, AG, would ha\e to be somewhat more than the aperture of the 

 objective. If it were practical, its most appropriate application 

 would be in connection with the ocular of an erecting telescope as 

 an eyepiece prism, for the double reflection would preserve the 

 original erect position of the image; but the path of the emergent 

 beam through the prism, would be of such length that the 

 size of the field, as well as the illumination, would be \ery materially 

 reduced 



193 



