STUDIES IN SPECIAL SENSE PHYSIOLOGY 387 



qualitative connection or resemblance between the physical pro- 

 cesses of stimulation and the psycho-physiological changes asso- 

 ciated therewith in the sense organ and " consciousness." For 

 this reason, and although no definite proof of its applicability to 

 all cases of sensory stimulation has been furnished, the law marked 

 an important step forward in the study of physiological psychology. 



SECTION II. NORMAL VISUAL STIMULI 



The physical basis of experimental work on colour stimulation 

 is to be found in the conception of a simple or homogeneous light, 

 and dates from Newton's researches in prismatic analysis. By 

 homogeneous light we understand ethereal vibrations all having 

 the same wave length or vibration frequency (within assigned 

 limits). Prisms and gratings allow us to filter such lights from a 

 mixture and employ them for our experiments. A pure light is 

 uniquely defined by its wave length, and any such light may possess 

 any intensity. Unfortunately, the physical unit of intensity is not 

 readily fixed. 1 



In any spectrum, the intensities of individual lights depend on 

 the source of illumination and the method of analysis i.e. on the 

 extent of surface over which light of a given wave length is dis- 

 persed. In an interference spectrum, dispersion is uniform ; in a 

 prismatic spectrum, on the other hand, it increases from red to 

 violet, so that the short-waved light is relatively less intense than 

 the long- waved. Spectra obtained by the two methods are not 

 therefore directly comparable. 



By colour or light mixing we understand an arrangement by 

 means of which two or more homogeneous lights fall upon the 

 same retinal area. Numerous experimental methods have been 

 devised for this purpose, and the results obtained 2 enable us to 

 formulate certain general statements respecting chromatic stimuli. 



lie application by Krarup (H. Krarup, Pkysiach-opthalmologixhe ', 

 prubleme, Leipzig, 1906) of Angstrom's energy measurements is a step in the 

 right direction. 



1 As this essay is chiefly concerned with the theories of colour vision, I assume 



the reader to be acquainted with the main experimental data and methods. Such 



knowledge can be obtained by consulting any good text-book, for instance Dr. 



article in the second volume of Schafer's Text-book, or Prof. v. Kries' article 



in the third volume of Nagel's J/amlbuch. 



