106 Dr. Draper's Further considerations 



and a photometer can be obtained which gives results compa- 

 rable with those of the tilhonometer : such an instrument I 

 have constructed ; it is exceedingly simple, as the following 

 considerations prove. 



We are to remember that the tithonometer gives indications 

 which are expressive of ihe intensity of the blue rays gene- 

 rally ; the blue tithonic rays are the rays which it measures. 

 In using the term blue, it will be understood to comprehend 

 the blue, indigo and violet, or the more refrangible rays ge- 

 nerally. It is obvious, therefore, that the photometer which 

 js 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 comprehended between the same limits of re- 

 frangibility. 



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 titho- 

 nometer is engaged. It is fortunate, as I have found, that 

 such a medium occurs in a solution of sulphate of copper and 

 ammonia. 



Let a wooden box, six inches long, two wide and two deep, 

 be provided ; in the centre of its top an aperture three quar- 

 ters of an inch in diameter is to be made ; the box must be 

 blackened interiorly, and a rectangular prism of wood 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 photometer, as described in the Annals of Philoiso- 

 phy. Upon the aperture in the top of the box a glass trough 

 is placed; it is made by drilling a circular hole, an inch in 

 diameter, 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 solution of 

 sulphate of copper and ammonia may be inclosed ; over the 

 trough a conical tube six or eight inches long is placed, so 

 that the eye may see distinctly through the aperture in the top 

 of the box the disc of paper, and more especially its dividing 

 diameter. 



Two small lamps are then prepared, of such dimensions that 

 when set opposite the open ends of the box their rays may il- 

 luminate the paper; they are supposed to be adjusted so as to 

 shine with equal intensity. 



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 experiment this pre- 



