1891.] Lord Bayleigh on some Apjplications of Photography. 261 



WEEKLY EVENING MEETING, 

 Friday, February 6, 1891. 



Sir Frederick Bramwell, Bart. D.C.L. F.E.S. Honorary Secretary 

 and Vice-President, in tlie Chair. 



The Eight Hon. Lord Katleigh, M.A. D.C.L. F.E.S. 3IM.L 



PROFESSOR OF NATURAL PHILOSOPHY R.I. 



Some Applications of Photography. 



One of the subjects to which I propose to invite your attention this 

 evening is the application of instantaneous photography to the 

 illustration of certain mechanical phenomena which pass so quickly 

 as to elude ordinary means of observation. The expression " instan- 

 taneous photography " is perhaps not quite a defensible one, because 

 no photography can be really instantaneous — some time must always 

 be occupied. One of the simplest and most commonly used methods 

 of obtaining very short exposures is by the use of movable shutters, 

 for which purpose many ingenious mechanical devices have been 

 invented. About two years ago we had a lecture from Prof. Muy- 

 bridge, in which he showed us the application of this method — and 

 a remarkably interesting application it was — to the examination of 

 the various positions assumed by a horse in his several gaits. Other 

 means, however, may be employed to the same end, and one of them 

 depends upon the production of an instantaneous light. It will 

 obviously come to the same thing whether the light to which we 

 expose the plates be instantaneous, or whether by a mechanical 

 device we allow the plate to be submitted to a continuous light for 

 only a very short time. A good deal of use has been made in this way 

 of what is known as the magnesium flash light. A cloud of magnesium 

 powder is ignited, and blazes up quickly with a bright light of very 

 short duration. Now I want to cqmpare that mode of illumination 

 with another, in order to be able to judge of the relative degree of 

 instantaneity, if I may use such an expression. We will illumine for 

 a short time a revolving disc, composed of black and white sectors ; 

 and the result will depend upon how quick the motion is as compared 

 with the duration of the light. If the light could be truly instan- 

 taneous, it would of necessity show the disc apparently stationary. 

 I believe that the duration of this light is variously estimated at 

 from one-tenth to one-fiftieth of a second ; and as the arrangement 

 that I have here is one of the slowest, we may assume that the time 

 occupied will be about a tenth of a second. I will say the words 

 one, two, three, and at the word three Mr. Gordon will project the 



