208 A NEW PHOTOMETER. 



creased a hundred-fold. We are required to measure the intensity of light which has 

 passed through all sorts of absorbent media, and, therefore, has become excessively 

 coloured. How shall we compare together the rays which have gone through sulpho- 

 cyanate of iron, and are of a deep blood-red, with those that have passed through sul- 

 phate of copper, and are of a bright blue ? 



951. Nevertheless, this problem is capable of a complete solution, and a photometer 

 can be obtained which gives results comparable with those of the tithonometer : such 

 an instrument I have constructed ; it is exceedingly simple, as the following considera- 

 tions prove. 



952. We are to remember that the tithouometer gives indications which are expres- 

 sive of the intensity of the blue rays generally ; the blue tithonic rays are the rays which 



I it measures. In using the term blue, it will be understood to comprehend the blue, in- 

 digo, and violet, or the more refrangible rays generally. It is obvious, therefore, that 

 the photometer which is to be used with it must measure the same blue rays, or, in 

 other words, the tithonometer and the photometer must be affected by rays compre- 

 hended between the same limits of refrangibility. 



953. This can be effected by interposing in the photometer some absorbent medium, 

 which will admit no rays to pass it except such as are in the limits of refrangibility 

 with which the tithonometer is engaged. It is fortunate, as I have found, that such a 

 medium occurs in a solution of sulphate of copper and ammonia. 



954. Let a wooden box, a b {fig. 131), six inches long, two wide, and two deep, be 

 provided ; in the centre of its top an aperture three quarters of an inch in diameter is 

 to be made ; the box must be blackened interiorly, and a rectangular prism of wood, c, 

 be placed in the box, with its right angle in such a position that its edge bisects as a 

 diameter the circular aperture ; over this wooden prism a piece of clean white paper 

 should be pasted, care being taken that, where it bends over the right angle of the 

 prism, it is folded sharp. So far the reader will recognise in this RITCHIE'S photome- 

 ter, as described in the Annals of Philosophy. Upon the aperture in the top of the 

 box a glass trough, g h, is placed ; it is made by drilling a circular hole an inch in diam- 

 eter in a piece of plate glass one third of an inch thick, and then placing on each side 

 of it a thin piece of plate glass. This forms a circular trough, in which a strong solu- 

 tion of sulphate of copper and ammonia may be enclosed ; over the trough a conical 

 tube, d, six or eight inches long, is placed, so that the eye may see distinctly, through 

 the aperture in the top of the box, the disk of paper, and more especially its dividing 

 diameter. 



955. Two small lamps, ef, are then prepared, of such dimensions that, when set op- 

 posite the open ends of the box, their rays may illuminate the paper ; they are suppo- 

 sed to be adjusted so as to shine with equal intensity. 



956. On looking through the tube a circle of blue light is seen, and, if the lamps are 

 shining equally, its two halves are equally bright. At the commencement of every ex- 

 periment this preliminary observation should be made, and, if necessary, the proper ad 

 justments secured. 



957. Suppose, now, it were required to know how much blue light is transmitted by 



