110 PEACTICAL PHOTO-MICBOGBAPHY 



If a small subsidiary mirror B be fastened on one edge of the 

 prism, roughly in the position as shown, but in such a way that 

 it does not materially cut off the incident ray, then another 

 narrow beam of light is reflected in the direction D. It is 

 evident that the small mirror is so placed that it does not reflect 

 a beam at right angles to the incident beam. The reflecting 

 system should be supported on any suitable stand, and should 

 be capable of adjustment about an axis situated at the centre 

 of the reflecting surface of the prism. This movement should 

 be controlled by a large milled head C, or the prism should be 

 supported on a ball-and-socket joint with universal motion. 

 It will be found that if the subsidiary reflected ray is thrown 

 on to any white surface, such as the ceiling or the wall of a 

 room, and its position indicated by reference cross-lines when the 

 main beam of light is in the optical axis of the microscope, it 

 is then not difficult, by carefully rotating the milled head C 

 or by moving the prism supports and by watching the spot 

 D, to keep the rays of light always truly in the axis of the 

 microscope. 



Some similar arrangement has been used for photographing 

 stars, and the writer has successfully worked with an arc lamp 

 in which this idea was utilised. It is obviously of the nature of 

 a makeshift arrangement, but yet it is one of those methods 

 that in the hands of a careful worker will give results in no 

 way inferior to the most expensive of heliostats. 



