1891.] on Some Applications of Photography. 267 



of a circuit in connection with tlie outer coatings of the Leyden jars,* 

 the whole arrangement being similar to that adopted by Prof. Lodge 

 in his experiments upon alternative paths of discharge. Fig. 8 will 

 give a general idea of the disposition of the apparatus. [Several 

 photographs of breaking films were shown upon the screen ; one of 

 these is reproduced in Fig. 7, Plate II.] t 



This work proved more difficult than I had expected ; and the 

 evidence of our photographs supplies the explanation, namely, that 

 the rupture of the film is an extraordinarily rapid operation. It 

 was found that the whole difference between being too early and 

 too late was represented by a displacement of the falling ball, 

 though less than a diameter, viz. ^ inch nearly. The drop which we 

 gave was about a foot. The speed of the ball would thus be about 

 100 inches per second ; therefore the whole difference between being 

 too soon and too late is represented by ^^^ second. Success is im- 

 possible, unless the spark can be got to occur within the limits of 

 this short interval. 



Prof. Dewar has directed my attention to the fact that Dupr^, a 

 good many years ago, calculated the speed of rupture of a film. We 

 know that the energy of the film is in proportion to its area. When 

 a film is partially broken, some of the area is gone, and the corre- 

 sponding potential energy is expended in generating the velocity of the 

 thickened edge, which bounds the still unbroken portion. The speed, 

 then, at which the edge will go depends upon the thickness of the 

 film. Dupre took a rather extreme case, and calculated a velocity of 

 32 metres per second. Here, with a greater thickness, our velocity 

 was, perhaps, 16 yards a second, agreeing fairly well with Dupre's 

 theory. 



I now pass on to another subject with which I have lately been 

 engaged, namely, the connection between aperture and the definition 

 of optical images. It has long been known to astronomers and to 

 those who study optics that the definition of an optical instrument 

 is proportional to the aperture employed ; but I do not think that 

 the theory is as widely appreciated as it should be. I do not know 

 whether, in the presence of my colleague, I may venture to say that 

 I fear the spectroscopists are among the worst sinners in this respect. 

 They constantly speak of the dispersion of their instruments as if that 

 by itself could give any idea of the power employed. You may 

 have a spectroscope of any degree of dispersion, and yet of resolving 

 power insufficient to separate even the D lines. What is the reason 

 of this ? Why is it that we cannot get as high a definition as we 

 please with a limited aperture? Some people say that the reason 



* In practice there were two sets of three jars each. 



t The appearance of the breaking bubble, as seen under instantaneous 

 illumination, was first described by Marangoni and Stephanelli, Nuovo Cimento, 

 1873. 



