16 REPORT — 1860. 



this distance is j 4 _ ■ inch; the mean velocity being 1505 feet per second, so that 



each particle makes 8,077,200,000 collisions per second. 6th. That diffusion of gases 

 is due partly to the agitation of the particles tending to mix them, and partly to the 

 existence of opposing currents of the two gases through each other. From experi- 

 ments of Graham on the diffusion of olefiant gas into air, the value of the distance 



described by a particle between successive collisions is found to be 389000 of an 

 inch, agreeing with the value derived from friction as closely as rough experiments 

 of this kind will permit. 7th. That conduction of heat consists in the propagation of 

 the motion of agitation from one part of the system to another, and may be calcu- 

 lated when we know the nature of the motion. Taking ^ ^ of an inch as a pro- 

 bable value of the distance that a particle moves between successive collisions, it ap- 

 pears that the quantity of heat transmitted through a stratum of air by conduction 



would be luuuouuo of that transmitted by a stratum of copper of equal thick- 

 ness, the difference of the temperatures of the two sides being the same in both 

 cases. This shows that the observed low conductivity of air is no objection to the 

 theory, but a result of it. 8th. That if the collisions produce rotation of the parti- 

 cles at all, the vis viva of rotation will be equal to that of translation. This relation 

 ■would make the ratio of specific heat at constant pressure to that at constant volume 

 to be 1-33, whereas we know that for air it is 1*408. This result of the dynamical 

 theory, being at variance with experiment, overturns the whole hypothesis, however 

 satisfactory the other results may be. 



On an Instrument for Exhibiting any Mixture of the Colours of the 

 Spectrum. By Professor Maxwell, F.R.S.E. 



This instrument consists of a box about 40 inches long by 1 1 broad and 4 deep. 

 Light is admitted at one end through a system of three slits, of which the position 

 and breadth can be altered and accurately measured. This light, near the other end 

 of the box, falls on two prhyns in succession, and then on a concave mirror, which 

 reflects it back through the prisms, so as to increase the dispersion of colours. The 

 light then falls on a plane mirror inclined 45° to the axis of the instrument, and is 

 reflected on a screen in which is a narrow slit. On this screen are formed three pure 

 spectra, the position and intensity of each depending on the position and breadth of 

 the slit through which the light was admitted. The portions of these spectra which 

 fall on the slit in the screen pass through, and are viewed by the eye placed close 

 behind it. A colour compounded of these three portions of three different spectra 

 is seen illuminating the prisms, and can be compared with white reflected light seen 

 past the edge of the prisms. The advantage of the instrument over that described 

 to the Association in 1859 is, that by the principle of reflexion the rays return in the 

 same tube, so as not to require two limbs forming an awkward angle ; while at the 

 same time, by doubling the dispersion, the necessary length of the instrument is 

 diminished. By means of this instrument many observations of colours have been 

 taken. Some of these by a colour-blind person are published in the ' Philosophical 

 Transactions ' for I860. 



Further Researches regarding the Laivs of Chromatic Dispersion. 

 By Mungo Ponton. 



In this paper the author has revised, and improved in its details, his method of 

 expressing the refractive index of a medium as a function of the wave-length. 



He employs X to denote the ratio of any particular wave-length referred to that 

 of the fixed line B as unity. The numerical values of the wave-lengths of the lines 

 C, D, E, F, G, H are given, as calculated from Fraunhofer's measures. 



The author's formula for expressing the refractive index (/0 as a function of X is 



X n 

 /*=w , 



