MANUFACTURE OF GLASS FOR OPTICAL PURPOSES. 
to render whatever of this character it may have, imperfect, for the sake of 
giving to it a more abbreviated and popular form, would have been doing 
injustice to the objects and motives of those who have instituted and supported 
the experiments. 
§ 1. Process of Manufacture, 8$c. 8$c. 
1 . The general properties of transparency, hardness, and a certain degree of 
refractive and dispersive power, which render glass so valuable as an optical 
agent, are easily obtained : but there is one condition essential in all delicate 
cases of its application, which is not so readily fulfilled ; this is, a perfectly ho- 
mogeneous composition and structure. Although every part of the glass may 
in itself be as good as possible, yet without this condition they do not act in 
uniformity with each other ; the rays of light are deflected from the course 
which they ought to pursue, 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 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 ob- 
servable by the naked eye, it may easily be supposed to what an injurious ex- 
tent 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 erroneous re- 
presentation 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 striae themselves occasion harm, but there is every 
reason to believe that they rarely occur in glass otherwise homogeneous. Some- 
times, 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 
substance ; and at others, a bubble ascending may lift a line of heavy or more 
refractive matter into a lighter and less refractive portion above. But very 
often, and especially as glass is usually manufactured and collected for use, 
