30 



a moulded block, is due to some folding of the block during 

 the early stage of the moulding operation, and as a rule has 

 no real effect in distorting the passage of the beam through 

 the block. It is, however, an unsightly defect, which may 

 possibly obstruct an appreciable amount of light, and one 

 which a glass manufacturer would not think of expecting a 

 purchaser to tolerate. 



Occasionally small " feathers," isolated from the outside 

 of the block, are met with. These are due to the imperfect 

 closing up during moulding of a flaw, which was not detected 

 in the glass previous to this operation, thus resulting in the 

 imprisoning of a small amount of air as a sheet of very 

 minute bubbles. 



However desirable may be the optical properties of a 

 glass, it is unsuitable for use if it will not withstand the 

 action of the various climatic and other influences to which 

 it is subjected. 



6. The durability of optical glass is a subject in connec- 

 tion with which much work has been done, with rather 

 uncertain results. The only completely satisfactory method 

 of testing the durability of a glass is the subjection of the 

 glass to exactly those conditions under which it will be used 

 and for such length of time as the instrument may be expected 

 to resist deterioration of its surfaces. This being in almost 

 every case a test which it is impossible to apply, it becomes 

 necessary to hasten up the " weathering " of glass surfaces, 

 or to devise some other kind of test, in order to achieve 

 results within a reasonable time. 



Great attention was paid by Schott & Gen to what is 

 known as the " iodeosin " test, which consists in measuring 

 the extent of alkaline efflorescence which occurs when a clean 

 surface of the glass is exposed for a given length of time, 

 under certain conditions, to the attack of water contained in 

 a moist ether solution. 



It has, however, been shown that this test by no means 

 strictly measures the amount of deterioration of a glass 

 surface which will occur in the ordinary conditions of use; 

 at the present time greater importance is attached to the 

 " autoclave " test, which also has the advantage that it can 

 be carried out readily by one who is unskilled in delicate 

 chemical manipulations. 



In this test, the glass samples are heated in steam at a 

 pressure of 2 to 4 atmospheres for a few hours (no specified 

 pressure and time have been chosen yet as standards by a 

 competent authority), and it is then found that the surface 



