on the Existence of a Fourth Imponderable. 105 



This doctrine, however, will not, I am persuaded, stand 

 the test of criticism ; there are facts, and those very imposing 

 ones, which make it utterly untenable. It is my object in this 

 paper to set forth that evidence and offer further and clearer 

 proof of thephysical independenceof the tithonic rays and light, 

 and indirectly establish the existence of a new imponderable. 



The true issue of the question, as has been said, rests in 

 proving a clear distinction between light and the tithonic rays; 

 the other imponderables may be left out of the argument. 

 The mechanical properties of the two agents are so closely 

 alike, reflexion, refraction, polarization, interference, &c. 

 taking place under the same laws for both, that the discussion 

 necessarily becomes one of quantity and measure. Will a 

 given ray of light, disturbed by the action of absorptive media, 

 change its luminous and chemical relations pari passu? or can 

 we alter the one and leave the other untouched ? or, changing 

 both by any process of treatment, do both change to the same 

 extent ? 



The final decision of this question obviously rests in obtain- 

 ing accurate measures for the rays of light and for the tithonic 

 rays. It is the comparison of those measures which is to settle 

 the point. 



In this Journal (Phil. Mag., Dec. 1843) I have described 

 an instrument, under the name of the tithonometer, which 

 gives indications by the production of muriatic acid from the 

 union of chlorine and hydrogen. This instrument is affected 

 chiefly by the indigo rays, or more correctly speaking, by those 

 rays which extend over the blue, indigo, and violet spaces of 

 the spectrum, having their maximum in the indigo. It is im- 

 portant that the reader should keep this fact in mind. 



Optical writers have been greatly embarrassed for want of a 

 photometrical instrument which can measure the intensity of 

 light : the chief difficulty in the way is the impossibility of 

 contrasting together lights that differ in colour. By all it is 

 admitted, that the eye is able to judge of the amount of illu- 

 mination of white surfaces, or the depth of shadows within 

 small limits of error, provided the rays compared are nearly 

 of the same tint. 



But in the discussion on which I am now entering this very 

 difficulty is increased 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 excess- 

 ively coloured. How shall we compare together the rays 

 which have gone through sulphocyanate of iron, and are of a 

 deep blood-red, with those that have passed through sulphate 

 of copper and are of a bright blue? 



Nevertheless this problem is capable of a complete solution, 



