886 



THE ELECTROCARDIOGRAM 



microscope AE, which passes through a hole in the magnet, or 

 records may be made by placing an arc lamp at G, concentrating 

 the light on the fibre by a lens F and throwing the shadow on 

 to a moving photo-sensitive surface. Fig. 92 shows diagram- 

 matically the arrangements of galvanometer and accessories for 

 photographing the fibre movements. The distances are given 

 in millimetres. 



The optical mechanism for producing the electrocardiograms 

 needs some mention. The camera is a light-tight box fitted with 

 a cylindrical lens and an arrangement whereby a sensitive photo- 

 graphic plate or film (or bromide paper) is made to travel at a 

 uniform speed past the narrow lens. The field of the objective 



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Fig. 93. — Electrocardiogram from lead II. and Phono cardiogram talcen simultaneously 

 from a normal subject. 



is projected by an eyepiece on to the lens, which focuses it as a 

 spot of light on the part of the sensitive surface exposed b}' the 

 sHt. The shadow of the fibre appears as a dark spot in this band 

 of light. Thus if the plate or paper be moved downwards normal 

 to the cylindrical lens, the whole surface will be exposed to the 

 action of the light except that portion protected by the shadow 

 of the fibre. The movements of the fibre are, as we have seen, 

 parallel to the plane in which the lens is set, and therefore when 

 the fibre moves towards the reader (in the diagram) the result 

 will be a corresponding alteration in the position of the shadow 

 spot. A continuous record of these positions is formed on the 

 moving sensitised surface. 



The record (Fig. 93) shows vertical and horizontal markings 

 as well as the electrocardiogram itself. The horizontal mark- 

 ings enable one to find by inspection the potential difference 



