PROCEEDINGS OF THE SOCIETY. 
275 
be a very serviceable one, especially as the various movements were 
capable of being easily corrected in the event of wear. 
Mr. G. C. Karop thought there was one objection which appeared 
inseparable from the form of foot, and that was that the milled head 
was so awkward to find when the stage was inclined. He thought if the 
foot was made curved instead of straight it could be got at much more 
easily. 
Mr. J. W. Lovibond read the following note on the Measurement of 
Direct Light : — “ Direct light presents a difficulty which does not occur 
with diffused light, as, whilst with the latter there is no difficulty in 
defining the general luminosity of the light in neutral tint units of 
known value, and also the preponderance of any particular colour ray 
in units of colour value ; in direct light there is a penetration of the 
red ray in excess of the other colour rays which prevents the use of the 
absorptive glasses which have been graded for diffused light. This 
disability is evidenced by a development of purity and brightness in the 
red ray as the light becomes reduced in intensity by the interposition 
of neutral tint glass standard units. The first impression is to attri- 
bute this phenomenon to a preponderance of the red ray in the light 
itself, or to imperfections in the absorptive glasses ; but when we con- 
sider that the red ray becomes conspicuous on the absorption of lights, 
which, under normal conditions produce a distinctive and different colour 
sensation, it is difficult to account for it on the score of red ray pre- 
ponderance. Again, considering that on the absorption of diffused light 
by means of the neutral tint standards no such preponderance in the red 
ray takes place at all, it can scarcely be attributed to imperfections in 
the glass standards. 
Therefore, for the purpose of investigation, let us as a working 
hypothesis, consider it as one of condition. This idea is suggested 
by work already done, it having been found that when direct light is 
transmitted through a diffusive medium, which apparently equally 
affects all the rays, the excessive penetration of the red ray is reduced 
in proportion as the diffusive medium is increased in density. An 
example occurs in nature when sunlight is transmitted through a white 
fog which is uncontaminated by smoke or other impurities ; as the 
density of the fog increases, the penetration of the red ray is lessened 
until it attains a colour equivalent with the other colour rays of the 
light ; in this condition it can be gradually absorbed to extinction by 
means of the glass standard neutral tint units without development of 
colour. 
The apparatus shown to-night is an attempt to measure the excess of 
penetration of the red ray by means of a graduated scale of artificial fog, 
which is made by lightly abrading a number of thin slips of colourless 
glass, and making combinations of them into a graduated scale of ‘just 
perceptible differences.’ 
The method for microscopical light is to intercept the direct light 
at the eye-piece with that diffusive combination which permits the total 
absorption of the transmitted light by means of the neutral tint glass 
standard units without abnormal development of the red ray. 
