MOVEMENT 49 



regulated to suit the average speed of the conveyance 

 the movement of which is to be ascertained, be it 

 carriage, locomotive, or boat, etc.* Wide as is the 

 range of movements capable of being recorded by 

 mechanical means, nevertheless there are, as we re- 

 marked before, cases in which this method ceases to be 

 applicable. It will be seen how valuable the employ- 

 ment of photography becomes in cases of this kind. 



Photography of the Movements of Lippmann's Electro- 

 meter. — In 1877, our colleague and friend Lippmann 

 had just invented his capillary electrometer, an in- 

 strument so marvellously sensitive that it was capable 

 of registering the slightest electrical variation that 

 occurred in living tissues. But for this purpose it was 

 necessary to make this electrometer a recording instru- 

 ment. This was managed by means of photography. 



As the column of the electrometer is exceedingly 

 fine, the movements must be observed under the 

 microscope. 



This column presents totally different appearances 

 under different conditions of illumination ; on a light 

 background it appears as a dark line ; on a dark back- 

 ground, when illuminated from the sides, it stands out 

 as a very bright line. This column is seen to elongate 

 and contract according to the direction and the in- 

 tensity of the current acting upon it. By receiving 

 its image on a photographic plate a very intense black 

 line will be obtained. If the sensitive plate is moved 

 at a uniform rate at right angles to the axis of the 

 column, all its variations in length will be apparent 

 in the image. 



The effect of moving the plate is that the image 

 of the column is no longer a simple line ; it is spread 

 out in the form of a band, the sinuous border of which 



* For the details of the employment of odography, see La Nature, 

 No. 278, September 28, 1878. 



