AND ON A SIMPLE PHOTOMETER. 365 



When this instrument had both its receiving surfaces lighted by mere day- 

 light, there were seen, on looking through it with both eyes, two equal round 

 spots, one-fourth of an inch in diameter, of a very pure blue colour. If the aper- 

 tures next the eyes were equal, these spots appeared of exactly the same tint and 

 intensity ; but if one of the" apertures was a very little smaller than the other, 

 the spot, viewed through the smaller aperture, appeared of a sensibly darker 

 shade, in so much as to impress the eye with an idea of difference of colour. 

 This peculiarity greatly aids the eye in judging when the two spots are of exactly 

 equal brightness. It is, of course, necessary to take care that both apertures are 

 smaller than the pupil of the eye. 



In order to determine how far the results to be obtained by this instrument 

 might be found to agree with those which may be obtained from the method of 

 equal shadows, the instrument was first employed to compare the light of the 

 moderator lamp with that of a wax candle (a short 6) ; and, after repeated trials, 

 it was found that the results of the comparison, when made with this instrument, 

 exactly agreed with those obtained by the method of equal shadows, the light of 

 the lamp proving to be 3£ times that of the candle. Indeed it is, if anything, 

 easier to judge of the perfect equality in brightness of the two blue spots, than of 

 the equal darkness of two shadows. 



Several preliminary trials were next made, with a view to obtain a series of 

 approximations to the relative sizes of the apertures to be employed. In making 

 these observations, the observer should sit in an easy, half-reclining posture, with 

 his back to the sun, so as to have the receiving surface exactly opposite to that 

 luminary ; his elbows should be rested steadily on the table ; and his hands placed 

 at the sides of his eyes, to screen them from extraneous light. Before looking 

 into the instrument, the eyes should be closed for a little time, to render them 

 more sensitive to the feeble blue rays; and care must be taken to have the two 

 images simultaneously visible without effort, so as to admit of their exact compa- 

 rison. For a good observation, a bright, cloudless, and perfectly calm day, 

 should be chosen ; and the sun should be at an altitude of about 45°, so as to 

 give the mean brilliancy of sunshine. 



Thursday the 10th of August 1855 having proved entirely suited for the pur- 

 pose, advantage was taken of it, to make the decisive observations, which were 

 commenced about 11 a.m., and occupied between 15 and 20 minutes. 



The diameter of the aperture fitted to the sky-tube was 0-083 inch. The sun- 

 tube was tried first with an aperture of 0*015, through which the sun-lit surface 

 appeared a little brighter than that lighted by the sky. It was next tried with an 

 aperture of 0*01375, and then the sky-lit surface appeared the brighter of the 

 two. Lastly, there was applied to the sun-tube an aperture of 0*014375, and 

 then no appreciable difference could be detected by the most steady gaze. These 

 three apertures were changed several times, and always with the same results. 



