METHODS OF STUDYING VISION IN ANIMALS 81 



be identical in all laboratories, and further to the fact that all 

 the above instruments are affected in their behavior by tem- 

 perature changes, it is necessary to provide for some method 

 of standardization which will make it possible to employ the 

 cell always under the conditions which obtain while the cell 

 is being calibrated. This method is briefly as follows: While 

 the sensibility of a given cell is being determined, it is noted 

 that under the influence of green light (X^5oo^/a) a deflection 

 of 60 mm. is obtained. The cell is then exposed to the total 

 radiation of the standard 8 c.p. incandescent lamp, w^hich is 

 removed to such a distance that the deflection produced is 

 also 60 mm. Next, the cell is exposed to another similar lamp, 

 burning as before at an accurately measured voltage, and this 

 lamp is also removed to such a distance as that at which a deflec- 

 tion of 60 mm. is obtained. This second lamp accompanies the 

 selenium cell. When it becomes desirable to use the cell again 

 it is connected with the galvanometer and potentiometer and 

 is exposed to the incandescent lamp which is burning at the 

 same voltage and at the same distance from the cell as at the 

 time of standardization. If then the galvanometer does not 

 record a deflection of 60 mm. its sensibility is varied until such 

 a deflection is obtained. However true it may be that the 

 manipulation of the selenium cell is a trifle complicated, it is to 

 be noted that this objection is more than off-set by the tre- 

 mendous sensibility placed at our disposal." 



The apparatus for measuring energy consists of the following 

 parts: (i) A seleniurii cell of 2,000,000 ohms resistance, supplied 

 by J. W. Giltay, Delft, Holland, through Arthur H. Thomas 

 Company, Philadelphia, for $20. (2) A Leech and Northrup 

 type H wall galvanometer with a low resistance coil (31 ohms), 

 supplied with telescope and scale, costing about $45. (3) Three 

 Leeds and Northrup resistance boxes (1-10,000 ohms) for alter- 

 ing the sensibility of the galvanometer and for use with the 

 selenium cell, costing about $24 each; these, with the galvano- 

 meter, can be had of the Leeds and Northrup Company, Phila- 

 delphia. (4) Four storage batteries, open glass cell chloride 

 accumulator, with CT type of element, 4" x 5", supplied by 

 Jas. G. Biddle, Philadelphia, at about $2.50 each. 



Professor Pfund has consented to standardize a few of the 

 selenium cells. If it is desired, the whole apparatus can be 



