1829.'] On the Manufacture of Optical Glass. 277 



light; and if two plates 8 or 10 inches long, one of the yellow 

 heavy glass and the other of crown glass, be looked through 

 edgeways, it will be seen in a moment that the crown glass 

 intercepts by far the most light. The colour of the glass is of 

 no consequence, otherwise than as causing a loss of light from 

 interception ; for the tinge which is cast over objects looked at 

 through a telescope constructed with it is scarcely perceptible 

 to the most acute eye, and quite unimportant. When to these 

 circumstances is added the reasonable expectation entertained 

 of removing a large proportion of the little remaining colour 

 by the use of purified silica (21), it need not be anticipated that 

 experience will prove the glass faulty in this respect. 



100. There is one very important action of the glass upon 

 light, however, which may perhaps interfere more with its 

 application, in telescopes at least, than any other, . e. its 

 reflective power. This is very strong in all the heavy glasses, 

 far stronger than in flint, and exceedingly surpassing the 

 similar power of crown glass. It is in proportion, as might 

 have been expected, to the refractive power and the density 

 of the glasses, all these properties increasing with the oxide 

 of lead. The loss of light occasioned by the reflexion from 

 the two surfaces of a plate through which a ray is passed, 

 appears to me to be greater than from the united action of 

 both colour and bubbles in a piece of glass 7 inches thick. 



I endeavoured to ascertain the comparative quantities of 

 light reflected by these heavy and other glasses, in some pho- 

 tometrical experiments made upon the principle of similar 

 shadows, measuring only the reflexion from the first surface 

 of the different glasses, that from the second surface being 

 destroyed. The ray was made incident in all the cases at an 

 angle of 45. It was obtained from a small single-wicked 

 lamp, a ; and when reflected, its intensity was measured by the 

 distance of a similar lamp, b t whose direct light cast the com- 

 parative shadow. The uniformity of the two lights, or at least 

 of their relation to each other, was established by trials before 

 and after the experiments with the reflecting surfaces, and 

 each surface was tried two or three times, at intervals, and in 

 a mixed manner; so that no anticipation of the result could 

 in any case bias the mind. The following Table shows the 

 results, small decimals being neglected : 



