368 Mr. W. Crookes on the Measurement of the 



along the bar to a convenient distance, depending on the intensity of the 

 light to be measured : the whole length being a little over 4 feet, each light 

 can be placed at a distance of 24 inches from the disk. The flame is then 

 sheltered from currents of air by black screens placed round, and the light to 

 be compared is fixed in a similar way on the other side of the instrument. 

 The whole should be placed in a dark room, or surrounded with non- 

 reflecting screens ; and the eye must also be protected from the direct rays 

 of the two lights. On looking through the eyepiece two bright disks will 

 be seen, probably of different colours. Supposing F represents the standard 

 flame, and E the light to be compared with it, the latter must now be slid 

 along the scale until the two disks of light, seen through the eyepiece, are 

 about equal in tint. Equality of illumination is easily obtained ; for as the 

 eye is observing two adjacent disks of light, which pass rapidly from red- 

 green to green-red through a neutral point of no colour, there is no diffi- 

 culty in hitting this point with great precision. It has been found most 

 convenient not to attempt to get absolute equality in this manner, but to 

 move the flame to the nearest inch on one side or the other of equality. 

 The final adjustment is now effected at the eye-end by turning the polari- 

 meter one way or the other up to 45°, until the images are seen without 

 any trace of colour. This will be found more accurate than the plan of 

 relying entirely on the alteration of the distance of the flame along the 

 scale ; and by a series of experimental adjustments the value of every angle 

 through which the bundle of plates is rotated can be ascertained once for 

 all, when the future calculations will present no difficulty. Squaring the 

 number of inches between the flames and the centre will give their ap- 

 proximate ratios ; and the number of degrees the eyepiece rotates will give 

 the number to be added or subtracted in order to obtain the necessary 

 accuracy. 



The delicacy of the instrument is very great. With two lamps, each 

 about 24 inches from the centre, it is easy to distinguish a movement of 

 one of them to the extent of 0*1 inch to or fro ; and by using the polari- 

 meter an accuracy considerably exceeding this can be attained. 



The employment of a photometer of this kind enables us to compare 

 lights of different colours with one another, and leads to the solution of a 

 problem which, from the nature of their construction, would be beyond 

 the powers of the instruments in general use. So long as the observer, 

 by the eye alone, has to compare the relative intensities of tint-surfaces, 

 respectively illuminated by the lights under trial, it is evident that, unless 

 they are of the same tint, it is impossible to obtain that equality of illu- 

 mination in the instrument which is requisite for a comparison. By the 

 unaided eye one cannot tell which is the brighter half of a paper disk, 

 illuminated on one side with a reddish, and on the other with a yellowish 

 light ; but by using the above-described photometer the problem becomes 

 practicable. For instance, on reference to fig. 1, suppose the disk D were 

 illuminated with light of a reddish colour, and the disk C with greenish 



