TRANSACTIONS OF SECTION A. 493 



2. Recording all components of motion, to time trains, and to give detailed 

 information respecting the state of a line. 



3. Recording all components of motion on a quickly running band of paper, to 

 test locomotives and carriages. With these records each vibration may be analysed 

 separately. 



FRIDAY, SEPTEMBER 13. 



The following Papers and Reports were read : — 



1. On the Quantity of Deposit of Silver produced by the development on a 

 Photographic Flute in terms of the intensity of Light acting. By Captain 

 Abnet, G.B., B.E., F.B.S. 



Allusion has already been made in ray address as to the law which connects 

 the transparency of deposit on a photofrraphic plate with the intensity of light. 

 In order to investigate the matter further it became necessary to know whether the 

 amount of deposit varied in terms of the intensity of light acting. It was found 

 that it was useless to attempt to weigli the deposit on any reasonable area, and resort 

 had to be made to an artifice to ascertain the fact. The supposition being made that 

 the amount of deposit was proportional to the intensity of light acting, it was evident 

 that if by any means a certain number of particles of silver or other material would 

 be scattered over a given transparent area, tlien double the number of particles were 

 scattered over an equal area, and so on, that the transparency of the areas should 

 obey the same law as that of the deposit obtained by development, viz. T' = T f""-^^ 

 where T is the total transparency, T' the transparency of an area in which .v is any 

 power of 2. 



Experiments were undertaken to investigate this point. Carbon particles in the 

 shape of Indian ink were suspended in water in such a manner that one amount of 

 water contained double the amount of particles that the ue.\t darkest contained, 

 and 80 on. Bibulous and uniform squares of paper were soaked in these and dried, 

 and the blackness of each three measured, with the result that it was found that 

 the law was obeyed. To add further inlormation to this fact, a gelatine plate was 

 exposed to light and developed, and the film tiien removed and dissolved up. Of 

 the emulsion thus formed different quantities Avere taken and added to a plain 

 gelatine solution, the total being made up to a fixed volume. The emulsions were 

 80 arranged that one contained double the quantity of particles of the next lightest, 

 and so on. These emulsions were then applied to glass plates of equal area and 

 dried. The transparency of these plates was then measured, and it was found a^ain 

 that the above law was obeyed. 



This being the case, it follows that the deposit of silver made by different 

 intensities of light varies directly as the intensity of light acting. This of course 

 within such limits that reversal of the image is not commenced, and that the film 

 is not at any part exhausted of the silver salt which can be reduced. 



2. On Pin-hole Photography. By Lord Rayleigh, LL.D., Sec.B.S. 



In the ' Phil. Mag.' for 1880 it was shown that a simple aperture is as effec- 

 tive as the best possible lens in forming an image, provided only that the focal 

 length (/) is sufficiently great. Conversely, if / be given, the aperture (2/-) may 

 be made so small that the use of a lens would give no advantage. Rut if /"be such 

 as is usually afforded by a camera, the admissible aperture, being very much less 

 than tliat of the pupil of the eye, is insufficient for reasonably good definition. 



If X be the wave-length of light, the point at which a lens may be dispensed 

 with is given, viz. 



^=¥ m 



