1829.] On the Manufacture of Optical Glass. 



ences, are, according to an expression sometimes used, due to 

 impurity. The glass, either of the streak or of the neighbour- 

 ing parts, would be equally good for optical purposes were it 

 all alike. It is the irregularity that constitutes the fault ; and 

 hence, in this respect, a particular composition is of very little 

 importance. As glass is always the result of a mixture of ma- 

 terials having different refractive and dispersive powers, it is 

 evident that striae must exist at one period during its prepara- 

 tion ; and the point required is not so much to seek for a differ- 

 ence of composition, or for those proportions which are found 

 by analysis to exist in specimens of tried and acknowledged 

 good glass, as to devise and perfect a process by which the 

 striae period should be passed over before the glass is finished 

 and the formation of fresh striae be prevented. 



4. Besides these, there are other faults in glass. Sometimes 

 it is said to be wavy, when it has the appearance of waves 

 within its mass ; but this is only a variety of that irregularity 

 which has just been explained as constituting, when in a 

 stronger degree, streaks and striae. Occasionally appearances 

 are observed in it, which seem to indicate a peculiar structure 

 of crystallization, or an irregular tension of its parts : these, 

 there is every reason to believe, may be avoided by careful an- 

 nealing. Again : the glass sometimes includes bubbles, which, 

 when small and numerous, render it what is called seedy. 

 Bubbles are not usually considered as of much consequence to 

 the performance of the glass, but objectionable only because of 

 their appearance when the glass is looked at, rather than when 

 looked through. They each act like a very powerful but very 

 small double convex lens of a rare substance in a very dense 

 medium, or as equally deep double concave lenses of glass 

 would do in air; they rapidly, therefore, turn the rays im- 

 pinging on them on one side, and occasion a loss of light, just 

 as so many opake spots would do. But as even when numerous 

 their united area may amount to only a very small proportion of 

 the area of the plate of glass required for a telescope, this loss 

 of light is usually of but little consequence. In practice, it is 

 said that no other real evil than such loss of light is dependent 

 on them. 



5. Of all these faults, that of the irregularity constituting 

 streaks, striae, and waves, is the most difficult to avoid, and the 



