of Flint-Glass at Incidences of total Reflection. 51 



tigate the point for themselves ; and I feel certain they will be 

 convinced of the superiority of the reflecting telescope for all 

 purposes, excepting perhaps the application to divided instru- 

 ments; and increased patronage will undoubtedly bring the 

 working opticians to attain a perfection in execution, which 

 there is now so little encouragement to seek for, from the f^.- 

 shionable prepossession in favour of achromatic telescopes/ 



As a question in physical optics, this subject merits an atten- 

 tion much beyond any which I have been able to give to it, 

 from the difficulty of obtaining a pure glass in sufficient bulk. 

 But though these experiments were only made with prisms of 

 common flint glass, and, as a matter of course, contained many 

 of those waves and striae to which it is subject, yet they give 

 us results which may be taken as proving the truth of the ge- 

 neral opinion, — that no light is lost in what are called total 

 reflections in transparent bodies ; and we should consequently 

 conclude that it is the same for all incidences at which this 

 effect takes place. 



If there is any variation in the intensity of the reflections, it 

 is evidently very small, and much more perfect prisms and 

 longer attention would be necessary to determine it. It is on 

 account of the imperfection in the glass that I have not so 

 multiplied the experiments as would otherwise have been de- 

 sirable, knowing that no decisive argument could be drawn 

 from them where the differences to be detected, if any, were 

 evidently so very small; and the experiments in photometry 

 with lamps present nothing very enticing and pleasant in them- 

 selves, and require, besides, considerable practice and patience 

 to get uniformly very exact results. 



The only correct method of proceeding in this inquiry is to 

 have the prisms formed so that the light may be incident and 

 emergent perpendicularly to the surfaces, and falling at the 

 required angle on the surface producing total reflection; and 

 also, which is of equal importance, that the thickness of glass 

 through which the rays pass may be the same in all. The 

 prisms I have used were similar in shape to fig. 1. 2. and 3, 

 where the distances a b, b c, are equal in all ; and by a rectan- 

 gular piece similar to fig. "t, where the length c c is equal to 

 the sum of the two lengths a b, b c in the prisms, we learn the 

 quantity of light transmitted under all similar circumstances, 

 excepting the total reflection, which enables us to complete 

 our deductions, by allowing for the loss attending a direct 

 transmission. 



The length a c, or the sum of the lengths n b, b c, was 1 '98 

 inch, and the other dimensions of the sections of the prisms 

 were proportionally as represented in the figures, the depths 

 of each being equal to the depth a e of the rectangular pieces. 



Third Series. Vol. 1. No. 1. Jidj 1S32. I 



