prove. He could and did show that a thermometer rose in passing from the violet to the red end of the spectrum: but he did more. He placed the thermometer beyond the visible red, and found that, as it continued to rise, heat-rays, invisible to the eye and less bent from the straight path of the sunbeam, gave the greatest heat. He must have asked himself. Is there not something similar at the violet end; but he had not the means of answering the question. He did what was next best. He asked a question pregnant with great results, and destined to bear an abundant harvest for the welfare and instruction of man. "It may be pardonable if I digress for a moment, and remark, that the foregoing researches ought to lead us on to others. May not the chemical properties of the prismatic colours be as different as those which relate to light and heat: . . . they may reside only in one of the colours." To this question he could neither give nor get an answer. A short time passed, and the answer came from Germany and, independently, from England. "The existence of solar rays accompanying light, more refrangible than the violet rays, and cognisable by their chemical effects, was first ascertained by Mr. Ritter." They were called "The dark rays of Ritter," and "appeared to extend beyond the violet rays of the prismatic spectrum, through a space nearly equal to that which is occupied by the violet." "Paper dipped in a solution of nitrate of silver" was used to prove the existence of these chemical rays and to introduce the days of photography. It was most fitting that it should be so. An astronomer led the way in this new quest after invisible rays; chemistry supplemented his discoveries
