56 Mr. Potter on the Reflection at the second Surface 



only by a rotatory motion about the axis would entirely de- 

 stroy the surface, by producing rings." 1 maintain that with 

 such a lateral motion no true figure of either an ellipse, a pa- 

 rabola, or an hyperbola, can be obtained ; and I have found in 

 my experience no ill result to arise from using, as I prescribe, 

 the rotatoiy motion entirely, whilst producing the change of 

 figure, even with the inferior dexterity which always accom- 

 panies an amateur hand. 



XV. ExperimcJits to determine the Heflection at the second 

 Surf ace of Flint Glass at Incidences at 'uohich no Portion of the 

 Ma7/s passes through the Surface. By R. Potter, Esq. Jun.* 



T^HIS subject is one worthy of attention on several accounts. 

 -*• A prism producing total reflection has been proposed as 

 a substitute for the small plane metallic mirror of the Newto- 

 nian telescope; and it is generally mentioned in all our optical 

 treatises, that considerable advantage would arise from the 

 substitution, without mention being made of the obvious at- 

 tendant disadvantages, which would lead to the idea that these 

 latter are trifling compared with the former. This would tend 

 to lead many to make an experiment, bringing some trouble 

 and expense, but in which they could experience no satisfac- 

 tory result. 



In telescopes of large size, where the prism in consequence 

 must be large, and the quantity of glass through which the 

 light passes proportionally so, there would, in addition to the 

 aberration and confusion introduced by the prism, be a disad- 

 vantage in point of light also, when the prism became above a 

 certain dimension, depending on the glass which was used. 

 This arises from the property, which all transparent bodies 

 possess, of absorbing a considerable portion of the light which 

 passes through them; and it is greater, even in the most trans- 

 parent of flint glass, than most persons have any conception 

 of. It will be seen, from the experiments about to be related, 

 that with prisms of only the size which I have employed, the 

 light reflected and transmitted does not greatly surpass that 

 reflected by a mirror of speculum metal which has been pro- 

 perly polished. The experiments furnish also a confirmation 

 of the fact, to which I have drawn attention in a former paper, 

 of the great quantity of light lost in achromatic object-glasses 

 of large size, from the unavoidable thickness of the material 

 through which the light traverses. 



I hope this consideration will induce astronomers to inves- 



• C'onimunicatcd hv the Author. 



