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II. On the Power of Glass of Antimony to reflect Light. By 

 R. Potter, Jan., Esq.* 



TN my paper on Comparative Photometry, which I read be- 

 ■ tore the Physical Section of the British Association for the 

 Advancement of Science at the Meeting at Oxford, and which 

 is published in the Philosophical Magazine and Journal of Sci- 

 ence for Sept.1832, I have given a table of the quantities of light 

 reflected by diamond for several incidences. I have since re- 

 peated the experiments for a low incidence on another larger 

 specimen, a triangular larke diamond, which Sir David Brew- 

 ster politely lent me for the purpose. These experiments in- 

 dicate that this diamond also reflects at 2° to 3° incidence 

 equally with crown glass at 64°, from which the various trials 

 differed very little. 



The small surfaces which we have on diamond cannot be 

 expected to be sufficiently plane to enable us to get results for 

 high incidences on which we may place entire confidence, and 

 the smallness of the surfaces, also, is an obstacle, even if they 

 were truly plane, to our obtaining quantities sufficiently exact 

 for determining the curve which these reflections form, when 

 we take the sine of incidence for the abscissa in rectangular 

 coordinates. 



These considerations induced me to make another attempt 

 (having failed in a former one,) to give a perfect polish to glass 

 of antimony, which has a refractive power but little inferior to 

 that of diamond. 



I used in this instance, for polishing powder, the fine pre- 

 pared oxide of iron which I have described the method of 

 procuring in a paper on the polishing, &c, of specula in the 

 Edinburgh Journal of Science. By using this powder on a 

 polishing bed of soft pitch, I succeeded completely ; and as I 

 had previously got the surfaces plane on a white German hone, 

 which had been very carefully prepared for making the small 

 specula of the Newtonian telescope, I had now a high refract- 

 ing substance, of which I could determine the reflective power 

 for any requisite incidences. The deep colour which glass 

 of antimony possesses is also an advantage for these experi- 

 ments, where we want only the reflection at the first surface. 



I have made two sets of trials, which are given in two Tables 

 below. Those in the second Table were obtained on a rather 

 more favourable morning for using the photometer than the 

 others. The incidences at which crown glass gave illumina- 

 tions equal to those at the required incidences for glass of an 



' 'ommunicatcd by the Author. 



