632 REPORT— 1900; 



dyers knovt Low persistent is the tendency to tlie development of ted in tlie 

 production of compound shades. 



The need of an artificial light which should so closely resemble daylight as to 

 show colours in their true relationship has long been felt by workers in colour. 

 At present the electric arc light is largely used for colour work, but, as we have 

 seen, it is far from satisfactory. 



The peculiar character of daylight is due essentially to the modification 

 produced by the atmosphere in the light from the sun. Light from a north sky 

 as usually adopted for colour work is deficient in red, orange, and yellow rays, 

 and consequently the light from a clear north sky is intensely blue. 



Starting with the electric arc light as being nearest daylight in character, we 

 have attempted to imitate by direct absorption the effect produced by scattering 

 in the atmosphere. 



The light of an arc lamp consists of two distinct parts : — (1) The light from 

 the glowing carbons ; (2) the light of the arc itself, characterised by its richness in 

 violet rays. In lamps of the enclosed arc type the length of arc is increased, and 

 consequently such lamps give a light richer in violet rays. Although arc lights 

 vary somewhat in the proportion of violet light, they all agree in being richer 

 than daylight in the amount of red, orange, and yellow rays, compared with the 

 amount of green and blue. Owing to the peculiar transparency of colours to red 

 light already noticed, it is of primary importance that the proportion of red light 

 should be carefully adjusted. Small variations in the amount of violet light are 

 of minor importance, owing to the eye being less sensitive to such rays, and also 

 because in mixing colours there is not the same tendency to develop a band of 

 violet as we have seen occurs in the red, since yellow colours generally have 

 complete absorption in the violet. 



The required absorption of the less refrangible rays can be effected by means 

 of blue cupric salts. A solution of copper sulphate shows strong absorption at 

 the extreme red of the spectrum, the absorption extending with diminishing 

 intensity into the green. 



For practical purposes the light from the arc is modified by passage through 

 pale blue glass coloured by means of copper. This coloured glass may conveniently 

 take the form of a globe replacing the ordinary globe of the arc light. 



FRIDAY, SEPTEMBER 7. 

 The following Papers were read ; — 



1. On the Statistical Dynamics of Gas Theory as illustrated by Meteor 

 Sivarms and Optical Rays, By Dr. J. Larmor, F.R.S. 



Imagine a cloud of meteors pursuing an orbit in space under outside attraction 

 ' — in fact, in any conservative field of force. Let us consider a group of the 

 meteors around a given central one. As they keep together their velocities are 

 nearly the same. When the central meteor has passed into another part of the 

 orbit, the surrounding region containing these same meteors will have altered in 

 shape ; it will in fact usually have become much elongated. If we merely count 

 large and small meteors alike, we can define the density of their distribution in 

 space in the neighbourhood of this group : it will be inversely as the volume 

 occupied by them. Now consider their deviations from a mean velocity, say that 

 of the central meteor of the group ; we can draw from an origin a vector repre- 

 senting the velocity of each meteor, and the ends of these vectors will mark out a 

 region in the velocity diagram whose shape and volume wUl represent the 

 character and range of the deviation. It results from a very general proposition 

 in dynamics that as the central meteor moves along its path the region occupied 

 by the group of its neighbours multiplied by the corresponding region in their 

 velocity diagram remains constant. Or we may say that the density at the group 



