f) 
MR. FARADAY ON THE 
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 ap- 
peared 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 frequently 
and to a much stronger degree with such as contain visible irregularities. 
3. It must not be imagined that striae, or those fainter differences, are, 
according to an expression sometimes used, due to impurity. The glass, 
cither of the streak or of the neighbouring 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 im- 
portance. As glass is always the result of a mixture of materials having dif- 
ferent refractive and dispersive powers, it is evident that striae must exist at one 
period during its preparation ; and the point required is not so much to seek 
for a difference 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 or crystallization, 
or an irregular tension of its parts : these, there is every reason to believe, may 
be avoided by careful annealing. Again: the glass sometimes includes bub- 
bles, which, when small and numerous, render it what is called seedy. Bub- 
bles 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 
