Page:On the various forces of nature and their relations to each other.djvu/191

take place with that large lens, when we throw the oil-lamp out of action, and put the electric light into use. It is astonishing to find how little the eye can compare the relative intensities of two lights. Look at that screen, and try to recollect the amount of light thrown upon it from the 3¾-inch lamp of Fresnel; and, now, when we shift the lens sideways, look at the glorious light arising from that small carbon point (fig. 58)—see how beautifully it shines in the focus of that lens, and Fig. 58.throws the rays forward. At present, the electric light is put at just the same distance as the oil light; and therefore, being in the focus of the lens, we have parallel rays which are thrown forward in a perfectly straight line—as you will see by comparing the size of