ON STANDABDS OF LIGHT. 41 



The Amyl-acetate lamp was compared witli the Standard Pentane 

 ^ame by placing the two equidistant from the screen. The flame of the 

 Amyl-acetate lamp was set at a height of 45 mm., and an observer 

 watching the two halves of the screen turned up and down the Pentane 

 flame until he judged the screen to be equally illuminated by the two 

 lights. The Amyl-acetate lamp gave a distinctly redder light than the 

 Pentane flame. In comparing the two lights four observers obtained 

 results which were appreciably different, each result being constantly 

 obtained in successive readings by the same observer. Two observers 

 made the light from the Amyl-acetate flame exactly equal to the normal 

 Pentane flame of 6S^ mm. ; a third observer made the Amyl-acetate flame 

 equal to a Pentane flame of 63 mm. ; and a fourth made it equal to a 

 Pentane flame of 62^ mm. 



On raising the Amyl-acetate flame to 50 mm. the same differences in 

 the estimation of the two coloured flames were found, but the change in 

 height of 5 mm. in the Amyl-acetate flame was compensated by a change 

 in height of 2 mm. in the Pentane flame. 



(5) The Pentane standard of Mr. Vernon Harcourt is reliable and 

 convenient, and fulfils all the conditions required in the adoption of a 

 standard of light. This standard attains this end by its having no wick 

 and consuming a material of definite chemical composition. The experi- 

 ments of your Committee also show that the light was not altered when 

 the specific gravity of the pentane was "632 or "628, instead of the specified 

 "value of -630. 



(6) Your Committee is not of opinion that the Pentane standard is 

 the only one which can be made possessing the necessary qualifications, 

 but it is the only one which has come under its notice, and it wishes most 

 earnestly to urge the importance of undertaking such action as is possible 

 to ensure the immediate rejection by the Board of Trade of the Parlia- 

 Tuentary candle as a standard, and the adoption in all future work of the 

 Pentane standard. 



(7) Your Committee wishes further to draw attention to the following : 



Appendix I. 



Photometric comparison of Candles, the Pentane Standard, the New Pentane 

 Lamp, and the Amyl-acetate Lamp. 



By the courtesy of Mr. Dibdin and the Metropolitan Board of Works 

 the testing-room at Spring Gardens was placed at the disposal of the 

 ■Committee for conducting experiments. The photometer employed was a 

 four-way one designed by Mr. Dibdin, the standard source of light in the 

 centre being a portion of the flame of an argand burning coal gas enriched 

 bj passing over pentane. This central flame was kept at a height of 

 about 3 inches, and only light from the middle of the flame was allowed 

 to fall on the photometer discs. In a preliminary series of experiments 

 made in January 1888 it was found that the light proceeding from the 

 central burner along each of the four photometer bars was equal. The 

 ^ttings and measurements of all parts of the photometer were also care- 

 fully verified by the Committee. 



It was decided that the actual tests should be performed by two of 

 the official gas-testers in the service of the Metropolitan Board. Messrs. 

 ■G. W. "Wood and R. Grimwood were selected by Mr. Dibdin to carry out 

 the work, which tbey performed in a most careful and satisfactory manner. 



