236 On the Manufacture of Optical Glass. [1829. 



striae, veins or tails, which are seen within glass otherwise per- 

 fectly good, result from a want of this equality ; they are visible 

 only because they bend the rays of light which pass through 

 them from their rectilinear course, and are constituted of a glass 

 having either a greater or a smaller refractive power than the 

 neighbouring parts. 



2. When these irregularities are so powerful as to render 

 their effects observable by the naked eye, it may easily be sup- 

 posed to what an injurious extent their influence must extend 

 in the construction of telescopes and other instruments of a 

 similar nature, where these faults are not only magnified many 

 times, but where the effect is to give an equally magnified erro- 

 neous representation of the object looked at, when the very 

 point to be attained is to examine that object with the utmost 

 accuracy ; and it is accordingly found that these striae are the 

 most fatal faults of glass intended for optical purposes. Besides 

 this, not only do the strige themselves occasion harm, but there 

 is every reason to believe that they rarely occur in glass other- 

 wise homogeneous. Sometimes, it is true, a grain of sand, in 

 passing through and at the same time dissolving in glass, will 

 give a streak of different composition to the rest of the sub- 

 stance ; and at others, a bubble ascending may lift a line of 

 heavy or more refractive matter into a lighter and less refrac- 

 tive portion above. But very often, and especially as glass is 

 usually manufactured and collected for use, striae are merely 

 the lines or planes where two different kinds of glass approxi- 

 mate ; and even if the striae could be covered so as to produce 

 no bad effect, yet the other parts, not being in every respect 

 alike, would exert an unequal action on light, and the piece be 

 therefore improper for the construction of a telescope. Many 

 a disc, which upon the most careful examination has appeared 

 perfectly free from striae and quite uniform, has, when worked 

 into an object-glass, been found incapable of giving a good 

 image, on account of the existence of irregularities in the mass, 

 which, though not sudden or strong enough to occasion striae, 

 still produce a confused effect ; and if this happens with glass 

 approaching so near to perfection, it happens still more fre- 

 quently and to a much stronger degree with such as contain 

 visible irregularities. 



3. It must not be imagined that striae, or those fainter differ- 



