Prof. Tyndall on Calorescence. 435 



less spherical body) without the help of the fluid-theory. New- 

 ton's first notions regarding the oblate form of the earth were 

 derived from reflections on fluid-pressure. The geodesist mea- 

 sures the lengths and amplitudes of his arcs, and, assuming that 

 they are of an elliptic form, determines on that hypothesis the 

 mean axes. Bat whence has he derived his conception that the 

 form of the earth may be an oblate spheroid ? From the fluid 

 theory, and nowhere else. We cannot do without the fluid theory ; 

 and to my own mind the evidence, especially from pendulum 

 experiments, and more particularly the influence of the bulging 

 out of the internal strata upon the pendulum (see from p. 85 to 

 97 of my Treatise), is quite overwhelming in favour of the con- 

 clusion that the distribution of the mass at present is proximately 

 according to the fluid-law. 



J. H. Pratt. 

 Calcutta, March 24, 1866. 



LXV. On Calorescence. 

 By Professor J. Tyndall, LL.D. Camb., F.R.S. fye. 

 [With a Plate.] 

 [Concluded from p. 396.] 



§ 4. TN the combination of bisulphide of carbon and iodine we 

 A find a means of filtering the composite radiation from 

 any luminous source. The solvent is practically transparent, while 

 the dissolved iodine cuts off every visible ray, its absorptive 

 power ceasing with extraordinary suddenness at the extreme red 

 of the spectrum. Doubtless the absorption extends a little way 

 beyond the red, and with a very great thickness of solution the 

 absorption of the extra-red rays might become very sensible. 

 But the solution may be employed in layers which, while com- 

 petent to intercept every trace of light, allow the invisible calo- 

 rific rays to pass with scarcely sensible diminution. 



The ray-filter here described was first publicly employed in 

 the early part of 1862*. Concentrating by large glass lenses 

 the radiation of the electric lamp, I cut off the visible portion of 

 the radiation by the solution of iodine, and thus formed invisible 

 foci of an intensity at that time unparalleled. In the autumn 

 of 1864 similar experiments were executed with rock-salt lenses 

 and with mirrors. The paper "On Luminous and Obscure 

 Radiation," already referred to, contains an account of various 

 effects of combustion and fusion which were then obtained with 

 the invisible rays of the electric light and of the sunf. 



* Philosophical Transactions, 1862, p. 67, note. 



t To the experiments there described the following may be added, as 



