530 STUDIES IN ADVANCED PHYSIOLOGY. 



eyes. Objectively this change in the form of the lens was 

 demonstrated by the reflection of images from the surfaces 

 of the lens, by L,angenbeck in 1849, and verified by Helm- 

 holtz in 1853. The ciliary muscles had been noted by 

 Briicke in 1846, and in 1856 Helmholtz attributed the 

 motor agency in accommodation to this muscle, and for- 

 warded the theory of accommodation as it still stands. The 

 innervation of this muscle was discovered by Hensen and 

 Volckers in 1868. The finer structure of the retina was 

 worked out by H. Muller's researches on the eye, 1855, 

 although as early as 1851 Helmholtz had designated the 

 rods and cones as the points sensitive to light. The blind 

 spot had been discovered by Mariotte as early as 1668. In 

 1876 the vision purple found in the rods was discovered by 

 Boll. 



Our knowledge of the physiology of color sensations has 

 been of slow growth. In 1657 the celebrated Newton in his 

 studies on the spectrum discovered the different refractive 

 indices of the different colors and their composition into 

 white light. In 1690 Huyghens proposed the wave theory of 

 light, while in 1746 Euler demonstrated that the difference 

 in colors depends on the rate of light vibrations, or what is 

 the same thing, the length of the waves. A theory of color 

 sensations was advanced by Young in 1807 which modified 

 by Helmholtz is to-day possibly still the best explanation at 

 hand. The Young-Helmholtz theory does not, however, 

 offer a satisfactory explanation for several observed facts, 

 and to include these a second entirely different theory was 

 advanced by Ewald Hering in 1872. Quite a number of 

 other observers have added to our knowledge on this sub- 

 ject. Such has been in briefest outline the path of the dis- 

 coveries which have led to our present understanding of the 

 eye. It may be in place at this point to add that there re- 

 mains very much still unexplained about the eye, and that 

 there are few subjects in physiology concerning which ad- 

 ditional information is more sorely wanted. 



