IN ITS RELATION TO THE EYE. 443 



be regulated by the amount of initial strain under which the eyes 

 are put to combine their images. The regulation of this ratio is 

 empirical and of importance; for, as is the case with the test for 

 loss of efficiency for clear seeing, the sensitivity of the test depends 

 to a considerable extent upon the initial value that is given to this 

 ratio. The eyes may be put under strain to combine their images 

 by interposing between them and the object viewed weak prisms 

 and by adjusting them and regulating the distance of the object 

 from the eye so that with the maximum of effort to see it as one, it 

 is seen alternately as one or as two in the proportion desired. 



We have also tested the tendency of different conditions of 

 lighting to produce ocular discomfort, and have explored the field 

 of vision for the purpose of determining the liability to discomfort 

 from the exposure of the eye to surface brilliancies of different 

 orders of magnitude. This tendency was measured by the time re- 

 quired for just noticeable discomfort to be set up, in the former case 

 both with the eye at work and at rest under the lighting conditions 

 in question, and in the latter with the eye systematically exposed to 

 a given area and brilliancy of surface at different points in the visual 

 field, by means of a large perimeter constructed especially for the 

 purpose. 



The following aspects of lighting sustain an important relation 

 to the eye: the evenness of illumination, the diffuseness of light, the 

 angle at which the light falls on the object viewed, the evenness of 

 surface brightness, the intensity of light, and its composition or color 

 value. For convenience of treatment in this paper we have grouped 

 the first four of these under the heading distribution factors. The 

 work throughout has been conducted primarily for the purpose of 

 finding out the comparative importance of these factors to the com- 

 fortable and efficient use of the eye rather than to test the merits of 

 various types and varieties of lighting. On the other hand, however, 

 the investigations have not been abstract in character. That is, all 

 the variations obtained were gotten in actual lighting situations by 

 employing so far as possible lighting installations in common use. 

 In order that a correlation might be had between lighting conditions 

 and effect on the eye, the following specifications of illumination 

 effects and conditions was made in each case. 



