JtTLT 17, 1914] 



SCIENCE 



89 



miich light should be used with a given 

 lighting system to give the best results for 

 Seeing ? " We have undertaken, therefore, to 

 determine the most favorable range of inten- 

 sity for the four types of distribution men- 

 tioned above. Curves have been obtained 

 showing the effect on the efficiency of the eye 

 of three or four hours of work under different 

 intensities of light, for the direct and semi- 

 indirect systems; and rough comparisons have 

 been made for the indirect system and for day- 

 light. Detailed tests will be made for these 

 latter two systems early next year. Our tests 

 show, in general, the following results. A very 

 wide range of intensity is permissible for day- 

 light and the indirect system. For the semi- 

 indirect system the eye falls off heavily in effi- 

 ciency for all intensities with the exception of 

 a narrow range on either side of 2.2 foot- 

 candles, measured at the level of the eye at 

 the point of work with the receiving surface 

 of the photometer in the horizontal plane. 

 For the direct system no intensity can be 

 found for which the eye does not lose a very 

 great deal in efficiency as the result of work. 

 Thus it seems that distribution is funda- 

 mental. That is, if the light is well distri- 

 buted and there are no extremes of surface 

 brightness as is the case for daylight and the 

 indirect systems of artificial lighting, the 

 ability of the eye to hold its efficiency is, 

 within limits, independent of intensity. In 

 short, the retina is itself highly accommoda- 

 tive or adaptive to intensity, and if the proper 

 distribution effects are obtained, the condi- 

 tions are not present which cause strain and 

 consequent loss of efficiency in the adjustment 

 of the eye. 



Details of the conditions of installation and 

 of the methods of working can not be given 

 here. It will be sufficient to state that the 

 work was done in the same room, with the 

 same fixtures, and in general with the same 

 conditions of installation and methods of 

 working as were used in the tests for distri- 

 bution. Nor can a full statement of results 

 be made. Time will be taken, however, for a 

 tQore detailed examination of the results ob- 

 tained for the direct and semi-indirect sys- 



tems. For the semi-ihdirect systems, our test 

 showed that the intensity most favorable to 

 the eye was secured when the photometric 

 reading with the receiving surface in the 

 horizontal plane showed 2.2 foot-candles of 

 light at the point of work, 1.52 foot-candles 

 in the 45° position, and .58 foot-candle in 

 the vertical position. At this intensity of 

 illumination, the semi-indirect system, so far 

 as its effect on the eye's loss of efficiency is 

 concerned, compares fairly well with the in- 

 direct system at such ranges of intensity as 

 we have employed. At intensities appreciably 

 higher than this most favorable value, or lower, 

 the loss of efficiency is very great. At the 

 intensity commonly recommended in lighting 

 practise, the semi-indirect system is almost, 

 if not quite, as damaging to the eye as the 

 direct system. The intensity recommended 

 by the Illuminating Engineering Society, for 

 example, in its primer issued in 1912, ranges 

 from 2-3 to 7-10 foot-candles, depending upon 

 the kind of work. Five foot-candles is taken 

 as a medium value. This medium value, it 

 will be noted, is more than double the amount 

 we have found to give the least loss of effi- 

 ciency for the type and installation of semi- 

 indirect system we have used. The intensity 

 we have found to give the least loss of effi- 

 ciency for this type of lighting, does not, 

 however, give a maximum acuity of vision 

 as determined by the momentary judgment. 

 At an intensity that does give maximal acuity 

 for the momentary judgment the eye runs 

 down rapidly in efficiency. That is, in this 

 type of lighting, one or the other of these 

 features must be sacrificed. High acuity and 

 little loss of efficiency can not be had at the 

 same intensity. They could both be had only 

 under the indirect system and daylight. How- 

 ever, the amount of light we find to give the 

 least loss of efficiency seems to be sufficient for 

 much of the work ordinarily done in the home 

 Or office. It is not enough, though, for draft- 

 ing or work requiring great clearness of 

 detail. 



In case of the direct system, we were able to 

 improve the conditions, so far as loss of 

 efficiency is concerned, by reducing the inten- 



