lilll] oil Biology and (he Cinematograph. 173 



speed as to produce u series of pictures wheii successive parts of the 

 film are exposed. 



A crank is turned about twice per second, and the works are so 

 geared that about sixteen fresh surfaces are exposed per second. 

 Briicke found that with black and white sectors on a disk the 

 greatest brightness corresponded to 17 'o stimuli per second, a result 

 confirmed by Burch. Higher rates, of course, are easily obtained, 

 but for ordinary work not less than sixteen pictures per second, the 

 height of each picture being 18 '5 mm., or | inch. Therefore, if 

 sixteen pictures per second — i.e. one foot of film — are passed before 

 the eye by means of the cinematograph " Projecting Apparatus " in 

 one second, the operator, if working without any interruption, runs 

 off sixty feet of film per minute. When the cost of the negative and 

 positive films is taken into account, an uninterrupted cinematograph 

 exhibition for ten minutes costs for film alone about £30, or £3 per 

 minute. 



Lendenfeld was one of the first, in 1903, to obtain two thousand 

 photographs per second, not by moving the sensitized surface, but by 

 displacing the object itself in relation to the sensitized surface. 



At the British Association Meeting held in Sheffield in September 

 last I had the honour of showing a number of films prepared by my 

 friend Dr. Comandon of Paris by means of what is called " dark 

 ground illumination," e.g., the movements of Paramecium, Trypano- 

 somes in blood, and the Microbes in the intestine of a mouse. I do 

 not propose to show you these films on this occasion. 



Dark Ground lUumination. 



Wenham by means of a parabolic condenser, more than half a 

 century ago, used this method to illuminate microscopic objects. 

 The light is not transmitted through the object on the stage of the 

 microscope, but is made to fall on the microscopic object and reflected 

 from it, so that the object appears bright on a black ground, a 

 method familiar to bacteriologists as well as to amateur microscopists. 



Wenham's dark ground illumination has led to that extraordinary 

 development of making the invisible visible, known as " Ultra- 

 Microscopy," and to instantaneous photography of Trypauosomes and 

 other transparent organisms in blood and other fluid, e.g. Spiro- 

 chetes. Brownian movements on a dark ground T saw some years 

 ago in the laboratory of my friend Professor Dastre in his laboratory 

 at the Sorbonne in France. 



Analysis of a Cinematograni. 



If one looks at a cinematograph film (e.g. an animal in motion) 

 by way of analysing the successive phases of a movement, one is 

 struck by what seems the impossible positions or attitudes of such an 

 animal in its onward progress — e.g. a galloping horse with its legs 



