Detecting Human Blood. corpuscles of each kind of blood which seemed most likely to be brought into ques tion in murder cases. He found the diame ters to be as quoted above. Then by means of the camera lucida, an attachment to his microscope, he cast the image of an aver age corpuscle of each variety onto a sheet of white paper, from which, with infinite care, he cut a disk exactly corresponding to it in size, but enlarged by means of com passes. He saw to it that his focus was absolutely the same while he carried on this work, and he knew when he had finished that he had six disks of paper which bore exactly the same relation to each other in point of size that the blood corpuscles did to each other. He then took these disks of paper, pasted them on glass, and used them as a lantern-slide. This enabled him to throw them on a screen magnified as many times as he chose. It would have been simple for him, had he so desired, to arrange an apparatus by which he could have made the smallest of them as large as the side of his study. He was contented, however,
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with magnifying them until the largest one measured about two feet across. It was then possible for him to take a foot-rule and measure the black spots on his screen with a certainty that the differences in size could not be affected by any small, extraneous influence. This method of cutting paper disks he selected as the most desirable, although at first it seemed that photography afforded the best means of accomplishing his ends. The adjustment of the photo graphic focus, however, is so delicate a mat ter, that he soon realized that this would add to the possibilities of inaccuracy, and therefore abandoned it. When Superin tendent Byrnes was told of Dr. Edson's new method, he greeted the news with pleasure. Said he : " Dr. Edson's discovery is most important. Few people will realize how important it is until they know that within a year, at least fifteen murder cases have oc curred in or near New York in which the identification of blood played a very import ant part."
