444 Direct Vision Spectroscopes by Double Internal Reflection. 



DIRECT VISION SPECTROSCOPES BY DOUBLE 

 INTERNAL REFLECTION. 



BY A. S. HERSCHEL, B.A. 



In the manufacture of prisms for spectroscopic purposes the 

 principle of double reflection, adopted in the construction of 

 the ordinary pocket sextant, can be employed for correcting 

 the deviation of the ray, which usually takes place in its 

 passage through a prism. This discovery, as it appears to be 

 of some importance, as well as novelty, will be treated in the 

 following article with rather close attention to mathematical 

 detail ; but it is hoped that it will not on this account be any 

 the less acceptable to readers of the Intellectual Observer. 



In the common sextant a 

 ray of light from a distant ob- 

 ject (a), falling in succession 

 upon two plane mirrors b c 

 (Fig. 1), is deflected from its 

 course in such a manner as 

 to be brought to coincide with 

 the direction of a ray of light 

 arriving from another distant 

 object, d; when both objects 

 become visible together in the 

 field of a small telescope at d. 

 The angular distance between 

 the two objects is then double 

 of the angle contained between the 

 two mirrors ; and is read off on 

 the sextant upon a scale of equal 

 parts, by a vernier and index 

 revolving with the mirror b. To apply this construction 

 to the removal of deviation from the refracted ray in spectro- 

 scopic prisms, two reflections of the ray must be made to take 

 the place of those of the pocket sextant, within the material 

 of the prism. These internal reflections are in general total, 

 and have the advantage over those of an ordinary sextant in 

 occasioning no loss of light. 



Suppose a prism of glass, a c d (Fig. 2), to be traversed, in 

 any manner, by a ray of light s a d e, with a deviating angle 

 amounting to 2 x a. This deviation will be completely removed 

 by intersecting the prism by a plane, a b, making the angle a 

 equal to half the deviation. At the same time the refracting 

 angle at c must be increased by the whole amount (2 a) of the 

 original deviation. The prism a c d is then replaced by a 

 prism of a new form, a c b, and the portion b d e of the 

 refracted ray is removed to c d e. The two internal reflections 

 revolve the ray within the prism through twice the angle (a). 



