172 Professor WilUam Stirling [May 12 



cameras, placed in different positions, were used to obtain movements 

 of the body and limbs from different points of view. Once the 

 pictures were obtained they were drawn on glass disks, which were 

 placed in the focus of a projecting lantern, and made by an ingenious 

 arrangement to rotate after the manner of a phenakistoscope, and 

 they were thus projected on a screen and the movement reconstituted. 

 Muy bridge called his apparatus a " Zoo-praxiscope." 



About 1881 Marey invented his ''Photographic Gun." In the 

 breech there were placed small sensitized glass plates, and a perforated 

 disk, which rotated twelve times per second, but was arrested inter- 

 mittently to allow of an exposure of y^^ of a second, i.e. the disk 

 came to rest twelve times in a second just opposite the beam of light 

 passing into the instrument. It was a very heavy apparatus, and 

 was used for the study of flight in birds. The pictures obtained by 

 Marey he combined in a zoetrope. 



Onimus and Martin, as long ago as 1865, used a photographic 

 plate to record the beats of the heart. The lens of the camera was 

 left open, and no shutter was used. The result was a double outline 

 representing the extreme phase of contraction and dilation of the 

 heart ; but there was no clear impression of the intermediate stages 

 — i.e. the " positions of visibility " were recorded at what are called 

 the " dead points," i.e. when the direction of the movement is 

 changed. 



Marey made extensive use of " fixed plates " for the recording of 

 such movements as jumping, walking, vaulting, fencing, carried out 

 against a dark background, the person making the movement being 

 dressed in white garments. 



Tlte Cinematograph. 



In the ordinary cinematograph a continuous narrow band of 

 flexible sensitized film moves behind the lens of the camera, and as it 

 moves it is checked for an instant and is stationary each time the 

 shutter is opened and the exposures made, and then the film moves 

 rapidly on in the intervals of darkness when the shutter is closed, so 

 as to expose a fresh surface of the film to liglit. The dissociation of 

 the successive images is obtained by moving the sensitized surface. 

 Usually about sixteen impressions per second are taken for ordinary 

 purposes. Even at this slow speed about 1000 impressions per 

 minute fall on the retina. For an animal running rapidly, or an 

 acrobat or cat turning somersaults in the air, forty to fifty impres- 

 sions per second suffice. 



The cinematograph is the modern evolution of chrono-photo- 

 graphy. For this purpose two pieces of apparatus are recpiired. 

 First, a " Producing Apparatus," or " Camera," to take impressions 

 of a moving object. A film of celluloid covered with sensitized 

 emulsion is moved behind a rapid short focus lens at such a uniform 



