762 ENERGY AND VISION 



media, for the total depth of the eye, which amounts only to about 

 1 per cent for 0.7 /x and less than 0.1 per cent below 0.65 /i, are entirely 

 negligible. The eye, for such small amounts of energy as those 

 corresponding to the minimum visibile, does not perceive a con- 

 tinuous increase in the brightness of the spot, when its intensity is 

 increased progressively, but seems to react by steps. High author- 

 ities, such as Joly, and Henri, disagree entirely as to the explanation 

 of vision on the basis of the quantum theory. 



Accuracy of the Method. — In the method used, the following causes 

 of error could be corrected : 



Errors Due to the Spectrometer. 



1. Selective reflection by the three gilded mirrors. 



2. Selective absorption by the rock salt prism. 



3. Contraction of the spectrum, (non-normal spectrum). 



Errors Due to the Use of a Nernst Filament. 



1. Uneven distribution of spherical energy. 



2. Disturbing effect of volt- and ammeter in the circuit of the 

 glower. 



3. Radiant output of glower, (losses by conduction). 



The following errors are also involved, and were not corrected: 



Errors Due to the Integration Method. 



1. Determination of limits of integration, (arbitrary). 



2. Material errors due to the mechanical integration of surfaces. 

 We can probably admit that they do not amount to more than 



10 per cent. 



Errors Due to the Assumed Quantity of Energy Radiated. 



1. Errors due to the fact that the image of the glower was not 

 formed on the collimator slit. 



2. Errors due to the emission of radiations from other parts of 

 the instrument. 



3. Errors due to the measurement of the total radiation by means 

 of a compensating current. 



