MANUFACTURE OF GLASS FOR OPTICAL PURPOSES. 29 



yet, unless it be of uniform constitution, the parts do not act 

 in conformity with each other ; the rays of light are deflected 

 from the course they ought to pursue, in passing through it, 

 and the piece of glass becomes useless. The streaks, striae, 

 veins, or tails, which are seen within glass otherwise perfectly 

 good, result from a want of this uniformity of constitution : 

 they are visible only because they bend the rays of light which 

 pass through them from their original rectilinear course, and 

 they do this because they consistof a glass having either a greater 

 or a smaller refractive power than the neighbouring parts. 



The effect of these irregularities being so powerful, when 

 observed only by the naked eye, it may easily be supposed to 

 what an injurious extent their influence must extend, in the 

 construction of telescopes, and instruments of a similar nature. 

 For in such instruments the faults I have described are not 

 only magnified many times, but the effect is to give an erro- 

 neous representation of the object viewed, with the error equally 

 magnified; when the very point which the observer is desirous 

 of attaining (and which it is necessary he should attain before 

 science can be promoted by his means) is to examine that ob- 

 ject with the utmost accuracy. It is accordingly found that 

 these striae are the most fatal faults of glass intended for opti- 

 cal purposes. 



The improvement required by science, it must be remem- 

 bered, is not merely an improvement upon the success ordi- 

 narily attained in producing glass in some measure fit for 

 optical purposes; it is the attainment, with respect to the con- 

 stitution of the glass, of absolute perfection, to obtain glass 

 as homogeneous and uniform in its nature as pure water, in 

 which every particle shall possess the same refractive power, 

 and the same relations to light in general. 



Now the striae, and the fainter differences of texture which 

 affect the utility of the glass, are not owing to impurity in its 

 composition. The glass, either of the streak or of the neigh- 

 bouring parts, would be equally good for optical purposes, 

 were it all alike. It is the irregularity which constitutes the 

 fault ; and hence, with respect to the existence or non-exist- 

 ence of striae, a particular composition is of very little import- 

 ance. As glass is always the result of a mixture of materials 

 which have different refractive and dispersive powers, and 

 which retain those different powers when melted, it is evident 

 that striss must exist at some period during its preparation. 

 Mr. Faraday, therefore, was not required so much to invent 

 a glass differing in composition from ordinary glass, or to pro- 

 duce one by combining the constituents of ordinary glass ac- 

 curately in those proportions which are found, by analysis, to 



