98 TRANSACTIONS OF THE [fFB. 21, 



lengthening the telescope. Bat the lenses of the 17th century 

 were very small and light, compared with the large lenses now 

 made. And it would be very difficult to mount rigidly a tele- 

 scope 100 feet long, carrying one of the large modern lenses. 

 Nevvcomb considers that, in the great refractors of recent times, 

 the limit of optical power for such instruments has been very 

 nearly attained. 



Many of the telescopes of the older astronomers had object- 

 glasses (of crown glass only) of from ^ an inch to 1 inch in diam- 

 eter. Some were larger. After Dolland made his discovery, the 

 great difficulty exi^erienced in obtaining discs of flint glass of the 

 required degree of purity prevented the making of any large 

 telescopes till the beginning of this century, when Guinand dis- 

 covered a process of making large discs of glass free from air- 

 bubbles and stri£e, and of equal density throughout. Most of 

 my audience understand that the object-glass of a refracting 

 telescope is the "vital part, the construction of which involves 

 the greatest difficulty." Given the object-glass, and the rest of 

 the telescope can be quite easily made. And in the making of 

 the object-glass there are two perfectly distinct processes. 

 First, there are the beautifully clear discs of crown and of flint 

 glass to be obtained. This is the work of the glassmaker. And 

 then these discs have to be ground and polished, so as to form 

 perfect lenses which will give uncolored images and bring all 

 the rays of light to one focus. This is the task of the optician. 

 Both require extraordinary skill. Few men have it. 



About the beginning of this century the "English Board of 

 Longitude" offered a considerable reward for bringing tiie art of 

 making flint glass for optical ])urposes to the requisite perfection; 

 but it led to no important discoveries. The Academy of Scien- 

 ces of Paris offered prizes in vain for this Object, and it remained 

 for a man, not distinguished by education nor a glassmaker by 

 trade, M. Guinand, of Switzerland, to have the honor of arriving 

 at the solution of the difficulties. 



Pierre Louis Guinand was one of those geniuses who seem to 

 have great intuition and immense perseverance. He is said to 

 have had no knowledge of optics, yet when quite young he con- 

 structed a small telescope equal to the best of his time. He soon 



