Barrett — Amount of Light Scattered from Rough Surfaces. 195 



order to exclude extraneous light, tlie observer rests his forehead in a sliaped 

 head-rest A, and a black cloth covers tlie head. After one minute the eye 

 attains a fah'ly steady state, and eitlier eye can be used at pleasure by sliding 

 the head-rest to and fro. 



On the glass bottom of H is a minute photograph of the graduated test 

 type used by oculists. This is viewed through a small lens F, adjustable at 

 E, until a sharp image is seen by the observer. When the cistern I is raised 

 until the glass bottom of H and I touch, the scale reading on L tlien 

 indicates zero. The depth of the liquid (as indicated on the scale) required 

 to produce complete extinction of the light measures the intrinsic brightness of 

 the source. Or with a constant source of light the depth measures the " light 

 threshold," or the sensibility of the observer's eye to light. This sensibility 

 rapidly rises during the first minute of observation, and becomes nearly 

 constant after two or three minutes. The form sense, or " visual acuity," of 

 the eye is measured by the depth of liquid required to obscure and produce 

 illegibility of the test type, and this also measures the illuminatinfj power of 

 the source of light. 



The illuminating power of the source may be reduced to any given 

 fraction by means of the adjustable and rapidly revolving sector, fig. 1, or 

 by other means ; and it will be found that the depth of liquid required to 

 produce extinction of the light is practically the same, even when the illumi- 

 nation from the source is reduced to a very minute amount : in other words, 

 the intrinsic brightness remains the same. On the other hand, the legi- 

 bility of the test type varies with the amount of illumination ; and it is this 

 we require to measure in the case of liglit irregularly reflected from rough 

 surfaces. 



Hence this arrangement affords an accurate method of testing the 

 illuminating power of any surface that scatters light, whether large or small. 

 It is only necessary to use a steady source of artificial light, and note the 

 depth of immersion of the plunger H which is required to produce illegibility 

 when a silvered mirror is employed ; then replace or cover the mirror by 

 a similar-sized piece of the reflecting surface to be tested, and note the depth 

 now required for extinction, the distance and intensity of the source of light 

 remaining unchanged. 



The following table gives the result of a few observations with this 

 arrangement. Calling the depth of liquid 100 when a silvered reflector is 

 employed, the percentages indicate the depth of the liquid required to 

 extinguish the legibility of a given line of test type, when liglit from 

 the same source is scattered from the various surfaces named. I have to 



