IN ITS RELATION TO THE EYE. 445 



tested was largely in excess of the mean variation or mean error 

 for each condition tested. This, the accepted conventional check 

 on the influence of variable extraneous factors was carefully ap- 

 plied at each step in the work. 



In attempting to make any presentation of results for a problem 

 so complicated as the one under investigation, in the space allotted, 

 we have had to choose between giving the details for some particular 

 piece of work and trying to draw some general conclusions from the 

 work as a whole, supplemented by an incomplete statement of data,^ 

 so far as the tests have been applied up to the present time. We 

 have chosen the latter alternative, although caution and our own 

 preference are on the side of the former. 



As already stated, the work has been in progress for six years. 



1 For a detailed statement of the data obtained in these experiments the 

 reader is referred to the Transactions of the Illuminating Engineering So- 

 ciety, 1913, VIII., pp. 40-60; 1915, X., pp. 407-447; 448-501; 1097-1138; 1916, 

 XL, pp. 1111-1137; 1917, XII., pp. 464-487. 



In these references will be found data on the following points for the 

 lighting conditions tested: (a) The horizontal, 45° and vertical components 

 of illumination at the 66 stations in the test room, the mean deviation of 

 these values from the average illumination and the percentage mean deviation 

 in some of the more important cases, (b) Measurements in candlepower per 

 square inch of the brightness of prominent objects in the room, such as the 

 test surface, the reading page, the ceiling spots above the reflectors for the 

 indirect installations; the reflectors and the ceiling spots above the reflectors 

 for the semi-indirect installations; the specular reflections from surfaces; 

 etc.; and the surfaces of lowest brightness to get the range, (c) Ratios be- 

 tween surfaces of the first, second, third, etc., order of brilliancy and surfaces 

 of the lowest order of brilliancy, and between surfaces of the first, second 

 and third order of brilliancy and the brightness at the point of work, to show 

 the gradations in surface brightness. Again in some of the more important 

 cases the mean deviation of the brightness values of the diff^erent surfaces 

 from the average brightness of all the surfaces measured, and the percentage 

 mean deviation have been given, (d) The angle of elevation of some of the 

 more important surfaces such as the reflector, opening of the reflector, etc., 

 above the plane of the observer's eye when in the working position, (e) 

 Photographs for each system of illumination representing to the eye the de- 

 tails of the test room, the location and type of lighting units, the position of 

 the test station, the apparatus with which the tests were made, the illumina- 

 tion effects (distribution of light and surface brightness), etc. And (/) 

 tables giving a detailed numerical statement of the results of the test includ- 

 ing among other items a comparison of the average error of each set of 

 determinations with the change of result produced by changing the lighting 

 conditions tested, as a check on the significance of the results. 



