206 



Dr. H. R. Wright on the Photometry of the 



substances that wave-length (X) was used which seemed most 

 brilliant in the spectrum. Wrong results may ensue from 

 an imperfectly homogeneous illumination of the surface. To 

 eliminate this I measured after each observation the starting- 

 point, and abandoned results which differed more than 5 per 

 cent. If less than 5 per cent, the geometrical mean of the 

 two values was used as a reference. 



The Compensation of the Measurements. — The measured 

 quantities are not the real objective-intensities of illumination ; 

 they are only functions of them, being the excitation produced 

 on the eye. According to Fechner's psycho-physical law the 

 intensity of the sensation is proportional to the logarithm of 

 the excitation. Consequently these logarithms must be used 

 for the compensation. Gauss's law of the least squares 

 leads to the following formula for the most probable value % 

 when by the different measurements for the same intensity 

 the values h l9 h 2 . . . h n are found, 



, _ log A t + log h 2 + + log hn. 



& x— n 



The following tables have been calculated in this way, 

 using 4, 6, or 8 single readings, and are given in the logar- 

 ithms themselves. The characteristic has been omitted as 

 being superfluous. 



The Results. 

 (1) English Red (F 2 3 ) : A,= 612 fifi. 



e. 



e=0°. 



^=20°. 



i=40°. 



i=W°. 



i=.80°.. 



1888 



-50° 











-40 











1838 



-30 









6710 





-20 









6931 



2592 



-10 







8861 















8838 



7294 



2978 



+ 10 





9734 









+20 





9508 



8532 



6955 



2650 



+ 30 



9357 











+40 



8788 



8562 



7630 



6109 



1762 



+60 



6804 



6750 



6123 



4283 



6156 



+80 



2397 



2203 



2044 



9133 



1435 



