January 28^ 1916] 



SCIENCE 



127 



density of image were obtained by Hurter 

 and Driffield twenty years ago. Since that 

 time much has been found out concerning 

 the nature of the latent image and of devel- 

 opment and the conditions which govern 

 speed and density gradient. The greater 

 part of the preliminary work in photo- 

 graphic research may be regarded as com- 

 plete. 



Investigation in photography is now cen- 

 tered upon the more recondite problems 

 and a clearing up of the nature of the pho- 

 tographic processes. We are not yet able 

 to express photographic density as a func- 

 tion of energy, wave-length and time. The 

 relation between plate speed and wave- 

 length is known for but few emulsions. 

 Speed varies with both absolute intensity 

 of radiation and the rate at which it is ap- 

 plied according to laws not yet understood. 

 The maximum density gradient obtainable 

 varies with exposure, wave-length, emulsion 

 and development in ways now being in- 

 vestigated. A knowledge of the resolving 

 powers of the photographic surface, of the 

 eye and of the lens or optical instrument 

 is of the utmost importance to workers in 

 almost every line of applied optics. 



The photographic emulsion is optically 

 a translucent medium of high scattering 

 power. The penetration of light into such 

 media varies a great deal with the wave- 

 length of the light, with size of grain and 

 with distance and direction in the medium. 

 Hence, a knowledge of the optical laws 

 governing scattered light is of the utmost 

 importance in photographic research and of 

 such laws but very little is known. 



The photographic reactions to light bear 

 a close resemblance in many respects to the 

 reaction of the retina and from the optical 

 properties of the retina much information 

 may be drawn that throws light on the pho- 

 tographic effects and vice versa. Hence, 

 the investigation of the retinal and photo- 



graphic reactions may very properly be 

 carried on side by side and results obtained 

 in either field applied in the other. 



Intimately related to all branches of ap- 

 plied optics are the visual properties of the 

 human eye. Broadly stated, what is known 

 of the eye as an optical instrument consti- 

 tutes but a rough working knowledge of it. 

 The curvatures, thicknesses and refractive 

 indices of the various eye media in an aver- 

 age normal eye are fairly well known as 

 well as the location of the nodal points and 

 center of rotation. Three of the third order 

 aberrations are important in vision, namely, 

 the spherical aberration, the chromatic va- 

 riation of the spherical aberration and the 

 axial chromatic aberration. Of these only 

 the last has been studied and measured and 

 that only recently. One remarkable result 

 of these measurements is the discovery that 

 many eyes possess a type of axial chromatic 

 correction previously unknown in lens op- 

 tics and which probably could not be dupli- 

 cated in a glass lens. It is to be hoped that 

 methods of. measuring the two other aber- 

 rations will shortly be devised and applied. 



The nature of the reactions of the retina 

 to light have been extensively studied dur- 

 ing the last twenty years. But the prob- 

 lems requiring investigation are many and 

 difficult and scarcely more than prelimi- 

 nary results have yet been obtained. The 

 visual impression requires time to originate 

 and grows at a rate varying with both the 

 intensity and wave-length of the light pro- 

 ducing it as well as with the previous treat- 

 ment of the retina. It is no simple matter 

 to isolate and measure these various effects 

 nor to correctly interpret the results ob- 

 tained. 



Most studied and best known is the rela- 

 tive brightness of the same amount of 

 radiation of various wave-lengths, the so- 

 called "Visibility" of radiation. This is 

 a measure of the relative sensibility of the 



