140 SCIENTIFIC NOTES. 



tlie hands of the best optician then in England, determined to go 

 to any expense to solve the question, Was it possible to make a 

 telescope so large as 25 inches diameter 1 All credit ought to 

 have been given to him for the spirit which led him to spend 

 £11,000 in so grand and so successful an experiment. Since 

 Cooke made that telescope others have not been idle. Alvan 

 Clarke and Sons, of Boston, have made one of 26 inches for 

 the Washington Observatory, one of 26 \ inches for a private 

 citizen of America, and have a flint lens now of 29 inches, 

 which only waits a fellow crown disc to be made up. Grubb, 

 also, of Dublin, has received an order from Vienna for one of 

 28 inches, and the only thing which hinders the production 

 of these large telescopes is the difficulty of making the' glass. 

 Practically there are only two makers — Chance Brothers, of Bir- 

 mingham ; and M. Feil, of Paris. I was anxious to know 

 something about the production of optical glass, and made it my 

 business to call upon both these makers, and I learned some- 

 thing that may be interesting to others. The special proportions 

 of ingredients to make the glass are of course trade secrets, but no 

 secret is or I suppose can be made of the way it is converted into 

 discs for optical purposes. I confess I had the impression that after 

 being made and allowed to anneal for a long time in the pot with 

 the greatest care, it was sawn into horizontal slices presumably of 

 uniform specific gravity, and after careful examination for faults, 

 ground into lenses. No such gentle handling, however, is accorded 

 to it, for after the pot of glass is cool, it is taken out and a good 

 sledge hammer applied to the pot, till it and contents fall into 

 fragments of all shapes and sizes. These are collected and sorted, 

 first by their sizes for special purposes, and then by careful exami- 

 nation as to quality ; larger pieces, if they contain any faults, 

 bubbles, sand grains, or pieces of pots adhering, have these at 

 once ground out if possible. If not, each piece is put into a clay 

 mould of the size it will fill, the bottom of the mould is curved 

 roughly to shape the glass, and then both together are put into the 

 oven until the glass is soft enough to be pressed gently into the 

 mould. After this it is carefully annealed. If any faults now ap- 

 pear they are ground out by a small emery wheel and the disc 

 moulded again. I have seen pieces that have been moulded five 

 times over in this way. It is not surprising, then, that many 

 of the discs so made are in a state of abnormal strain, and 

 exhibit the phenomena of polarization so much that they 

 have to be rejected, or again annealed, at the risk of breaking 

 them. Hence the enormous price obtained for such discs of 

 optical glass. M. Feil, of Paris, has the order for two discs 

 for Vienna, to be ground by Mr. Grubb, of Dublin. They are 

 to be 28 inches when finished ; and the moulds for them, which 

 M. Feil showed me, are 30 inches in diameter, 2 inches being 



