( III ) 



111 order (o ohlaiii llio iiiiuge of the string equally sharp in all 

 parts of the visual tiekl, the string must move in a plane perpen- 

 dicular to the optical axis of the projecting microscope. A displace- 

 meiit of the siring of 0.5^1 in the direction of the optical axis sulTices 

 to cause a noticeable indistinctness of the image with the magnifica- 

 tion used. The photograms show that such a displacement does not 

 take place. 



The great constancy of the zero point and the equality of the 

 deflections deserve notice and also — which is especially important 

 for practical work with the instrument in electro-physiological 

 measurements — the possibility of accurately fixing beforehand the 

 sensitiveness of the instrument. The unaided eye can already observe 

 in nearly all the figures of plate I that this can be done successfully 

 with an error of less than 0.1 mm. for deflections of 30 or 40 mm., 

 i.e. with an error of less than 2.5 or 3 per thousand. Only fig. 5 

 shows a real deficiency of about 0.1 mm. which some greater care 

 might have avoided. 



It is hardly necessary to point out that the galvanometer is 

 not affected by variations in the surrounding magnetic field. Moreover 

 it is not to any extent affected by tremors of the floor. It stands 

 on the same stone pillar on which a large tin disc with spokes is 

 rapidly rotated by an electromotor. This electromotor is only at a 

 few centimetres' distance from the galvanometer, while another 

 motor, coupled with a heavy fly-wheel, for moving the sensitive 

 plate, is clamped to the same pillar at a somewhat greater distance. 

 Yet no trace of mechanical vibrations appears in the photograms. 



The first electro-physiological investigation made with the string 

 galvanometer was one concerning the shape of the human electro- 

 cardiogram discovered by Aug. D. Wallek ^). Until now this could 

 only be obtained by means of the capillary .electrometer. But the 

 cune traced by that instrument gives, when superficially observed, 

 a quite erroneous idea of the changes of potential differences actually 

 occurring during the registering. In order to know these they have 

 to be calculated from the shape of the recorded curve and the pro- 

 perties of the capillary used. This leads to the construction of a new 

 curve, the form of which is the correct expression of the actual 

 variations of potential. 



^) Augustus D, Wallek. On the elecUomotive changes, connected with the beat 

 of tlic mammalian heart and of the human heart in particular. Philosoph. Trans- 

 actions of the Royal Society of London, vol. 180 (1809), B, pp. 169—104. 



8 



Proceedings Royal Acad. Amsterdam. Vol. VI. 



