PREFACE ^^-^^^ XV for ages been dwelt upon by the poets. But for a scientific explanation of this effect of sunlight in red- dening the blood we must turn to the spectrum analysis. The visible solar spectrum as shown through a prism by the ordinary sunbeam is made up of the seven different colours, namely, red, orange, yellow, green, blue, indigo, and violet. Instead of consisting simply of white light as a whole, it is now uni^rsally accepted that in this spectrum different properties belong to different parts. Light or lumi- nous power to one portion ; heat or calorific power to another ; and chemical power or actinism to a third. The visible solar or Newtonian luminous spectrum, resulting from the decomposition of white light by a prism, is only the middle portion of the whole solar spectrum. Beyond the red end there are rays pos- sessing still greater heating effect ; and beyond the violet extremity there are rays endowed with far more powerful chemical action. The violet, and especially these latter ultra-violet rays, redden the life stream by increasing the hsemoglobin — that crystal- lizable body which forms so large a portion of the coloured corpuscles of the blood. Sunlight, moreover, has not only this action upon the animal kingdom, but also upon the vegetable world as well. Plants, like celery, which are subjected to blanching, become whitened under the process of etiolation. This is due to the absence of chlorophyll,
