414 NATURAL SCIENCE. Dec. 



of mind and body. In the question of vision this difficulty is strikingly 

 obvious. We can see that light has a direct effect upon the proto- 

 plasm of many of the simplest animals and plants. Many single- 

 celled forms of life, imprisoned in a vessel, darkened, for instance, 

 by being covered with black paper except at one spot, will crowd 

 towards the window of light ; others will shun the light and assemble 

 in the darkest regions. In many of them, again, when they are 

 observed under the microscope by a beam of light reflected through 

 them by the mirror of the microscope, the direct effect of hght as a 

 stimulus may be seen by their sudden movements when the mirror is 

 flashed on them or turned off. From conditions so simple up to the 

 complicated eye of the mammalia an infinite number of grades of 

 complication in the arrangement of cells, specialised in the direction 

 of sensitiveness to light, may be observed, while, again, there are 

 similar series of grades of elaboration of accessory structures. Thus 

 there are structures that seem to be mechanisms directly optical, 

 like convex transparent lenses and corneas to focus the beams ; 

 curtains of pigment, perhaps corresponding to the velvet sheet of the 

 photographer ; pigments that are directly affected by the chemical 

 action of the light, like the sensitised film of a photographic plate. 



Some of these arrangements are fairly well understood ; about 

 the function and mechanism of others theories are numerous and 

 divergent. But were they all understood, not vision, but the 

 mechanism of vision, would be understood. The appreciation by the 

 brain or by the mind of the sensational stimuli afforded it by the 

 optical apparatus involves many things that are not optical. It 

 involves memories of previous visions, memories of touch, memories 

 of movements, and several other factors. The analysis of what we 

 know as sight, and the correlation of the component elements with 

 the results of chemical, physical, and physiological investigation of 

 the eye, is one of the obscurest tasks of the psychologist. 



Realism in Art. 



As an obvious example of the difficulties attending the pheno- 

 mena of vision, consider the dispute in art connected with what is 

 called reahsm. We may dismiss, first of all, idealised compositions, 

 pictures in which the imagination of the artist frankly transforms 

 his impressions of Nature. Take three types of paintings or draw- 

 ings, each of which are set forward by some and accepted by many 

 as adequate representations of what may actually be seen. To the 

 first of the three types belong the landscapes of Mr. Waller Paton, 

 scenic arrangements like Mr. Frith's celebrated " Derby Day," or 

 the later works of Sir John Millais. Round such as these one may 

 notice a group of admirers crying, " See, it is Nature itself ! Look at 

 the sheep on the distant hill with their owner's initial in red paint on 

 their fleeces, while on the grass-blades, in the foreground behold here 



