Buffon, a great naturalist, was followed by Cuvier, the first serious student of fossils; by Humboldt, naturalist and traveller; by Robert Brown, the founder of modern Botany; by Darwin and by Pasteur in turn. How much these men owe to the microscope can never be known; certain it is that without its assistance our world, the world we know and can see, would have been smaller than it is to-day. FOOTNOTE: [1] Humour is here used in its original sense, meaning moisture or a liquid. CHAPTER III THE ACTION OF LIGHT It is hardly necessary to remark that the wonderful properties of the microscope depend upon light. Without light, lenses would be useless, objects could not be illuminated and we could not see them. In this short chapter we propose to give a brief outline of the action of light; if our words appear to savour of the school-book, we shall try to avoid it, but, we repeat, if they do so we would remind our readers that the more one knows of the action of light the better use one can make of one’s instrument. As a well-known microscopist has remarked we may be able to afford a costly harp or a costly microscope, but although we may be able to strike a few notes on the former and examine a few objects with the latter, we can only make the best use of either by thoroughly understanding and practising upon it. The first thing we learn when we study light is that it travels in straight lines. The chief source of light to the inhabitants of this earth is the sun. Now the sun is so far away that, for all practical purposes, the rays of light coming from it may be looked upon as being parallel to one another. That we must always remember, when dealing with the sun, though, of course, it does not apply when we are dealing with lights near at hand, unless they are specially constructed to throw parallel beams or rays, whichever we elect to call them. To prove that light travels in straight lines is not difficult, and we may devise a number of experiments for the purpose. The doors and ventilators of many dark rooms, in which photographic operations are carried on, are constructed on the assumption that light cannot travel round corners. An arrangement as shown in the diagram will allow air, but no light, to pass. If light were capable of going round corners, some other arrangement would have to be devised for the ventilation of dark rooms. [Illustration]