OF OPTICS. 233 



SLndLBM; then, from the intersection A, with any LECT. 

 opening of the compasses, set off the equal arcs A D ^ S n> 

 and A E, and draw the right line DFE : then, taking 

 Fa, which is three quarters of the length F E, from the 

 point a, draw a I parallel to ABK, and join K I, pa- 

 rallel to B M: so K I will be equal to three quarters 

 of FE or of DF. This done, fix the board upright 

 upon the leaden pedestal O, and stick three pins per- 

 pendicularly into the board, at the points D, B, and /: 

 then set the board upright into the vessel T U V, and 

 fill up the vessel with water to the line L B M. When 

 the water has settled, look along the line D B, so as 

 you may see the head of the pin B over the head of the 

 pin D ; and the pin / will appear in the same right line 

 produced to G, for its head will be seen just over the 

 head of the pin at B : which shews that the ray I B, 

 coming from the pin at /, is so refracted at B, as to 

 proceed from thence in the line B D to the eye of the 

 observer ; the same as it would do from any point G in 

 the right line DBG, if there were no water in the 

 vessel : and also shews that K I, the sine of refraction 

 in water, is to D F, the sine of incidence in air, as 3 

 to 4." 



Hence, if D B H were a crooked stick put obliquely 

 into the water, it would appear a straight one, as D BG. 

 Therefore, as the line B H appears at B G, so the line 

 B G will appear at B g ; and consequently, a straight 

 fctick DBG put obliquely into water, will seem bent at 

 the surface of the water in B, and crooked, as D B g. 



When a ray of light passes out of air into glass, the 

 sine of incidence is to the sine of refraction, as 3 to 2 ; 

 and when out of air into a diamond, as 5 to 2. 



Glass may be ground into eight different shapes at 

 least, for optical purposes, viz. 



Note 65. This is strictly true of the red rajs only, for the other 

 coloured rays are differently refracted ; but the difference is so small, 

 that it need not be considered in this place. Aufe by the Author. 



