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NOTES ON THE EARLY EVOLUTION OF THE REFLECTOR. 



By Louis Bell. 



Received November 11, 1921. Presented November 9, 1921. 



There is some tradition of reflecting telescopes in classical times, 

 probably groundless save as the ancients had concave mirrors and 

 these, if of fairly long focus, would present to the eye an enlarged 

 image of a distant object. 



The first suggestion that a mirror might be used instead of an 

 objective seemed to have come from Mersenne about the middle of the 

 seventeenth century in a letter to Descartes. The idea did not 

 appeal to the latter and the matter was dropped. The first actual 

 reflecting telescope was designed by James Gregory, who published an 

 account of it in his "Optica Promota" in 1663. In this he described 

 the rather elegant construction which still bears his name, a perforated 

 parabolic mirror with an elliptical concave mirror placed beyond its 

 focus and returning an erect image to the ocular through the perfora- 

 tion. 



The next year Gregory started Reive, a well known London optician, 

 at making a 6 foot telescope. This failed on account of inability to 

 get the necessary figufe, largely owing to the fact that Reive tried to 

 polish his mirrors on cloth, never a satisfactory material for accurate 

 figuring. It is possible that Gregory had better luck later, for there 

 is a well defined tradition that he died from a stroke of apoplexy in 

 1675 while showing to a group of students Jupiter's moons through 

 one of his own telescopes. Certainly Robert Hooke presented a 

 passable example to the Royal Society at its meeting February 5, 

 1673^. 



Now enters upon the scene the biggest figure of the period in science, 

 Isaac Newton, then a young man who had just discovered the dis- 

 persion of light. A great speculative philosopher and mathematician, 

 he was neither a practical astronomer nor a particularly good experi- 

 menter, and failed to discover the dift'erence of relative dispersion in 

 refractive media by the inexcusable blunder of raising the refraction 

 of the water with which he was comparing glass by loading it with 

 sugar of lead, and then jumping at the conclusion that all substances 

 varied equally both in refraction and dispersion. 



