Internal Prismatic Reflexion. 355 



3. The central ray to pass through parallel to the base. 



Now, if any one of these conditions were altered in the 

 slightest degree, the relative points of emersion would be 

 changed, and the appearance of the spectrum altered. 



In order to shew this more clearly, I will trace the pro- 

 gress of a homogeneous ray (for the sake of simplicity) 

 through an equilateral prism, under six different circum- 

 stances, three depending on the point of immersion, and 

 three on the direction of its passage with respect to the 



As we do not know to what extent these reflexions may 

 go, I have traced them to twenty in each experiment ; now, 

 out of these twenty about seven will occur at the posterior 

 fece, and, as there is an emersion at every reflexion, there are 

 at the posterior face seven emersions, which form the sub- 

 ject of the following table ; the object of which is to shew 

 the difference in the direction and the intensity of the ray 

 occasioned by the six variations of the conditions before 

 stated. By intensity I mean priority in transmission or 

 emersion, for the numbers in the table denote the number 

 of reflexions which have preceded the respective emersions, 

 and a ray must obviously retain more luminosity after two 

 or three, than after twelve or sixteen reflexions. 



1 . Impinge equidistant from apex and base. 

 jnclined in its passage 5** towards base. 

 ;o")inclin 



direct ray, & 6= 5° (.^„^„\ 

 3&9= 5°)i,"'=""- 



Inclined 5° from base. 



"linclin. 

 3, 13, 19=5° [toward 

 3 base. 

 ,. , ^ . ^inclin. 



"^T'lfi^ =5° (from 



2. Impinge nearer to apex than to base by ^th. 



Inclined 5° towards base. 



"^inclin. 

 direct ray, 6, 16= 5° Vtoward 

 3 base. 

 ■Jinclin. 

 3,9,13, 19 = 5°Sfrom 

 3 base 



Inclined 5** from base. 



"Jinclin. 

 3, 12, 18=5° [toward 



3 base. 



direct ray 6) ^5o>'f;:^J;,"- 

 ^' '^> 3 base. 



2 A 2 



