398 SCIENCE PROGRESS 



phenomena attendant on the formation of shadows, the passage 

 of the sun's rays through windows and chinks, etc. Then it 

 deals with the necessity for the existence of gaps between the 

 rays ; we may look intently for a small object like a needle 

 lying immediately before our nose and not see it. This is 

 because it is lying in a gap between the rays. But if we direct 

 one of the rays from the eye on to it, we see it at once. The 

 same experiment proves that the raj's must go from the eye 

 to the object, because, if the needle itself were sending out 

 rays, we should see it all the time. 



Some authorities are of the opinion that the Optics and 

 Catoptrics were not written by Euclid, the geometer, on account 

 of the numerous errors they contain ; certainly the notes to 

 the Latin text by David Gregory, the Savilian professor at 

 Oxford, are very critical — the books have not been translated 

 into English. It is impossible to form a judgment on this point 

 without a special knowledge of the period, but to the amateur 



Fig. 2. 



in these matters there is an apparent strong similarity in style 

 between the geometry of the Elements and of the Optics and 

 Catoptrics ; possibly the latter were an account of Euclid's 

 teaching by another hand. But there is no doubt that the 

 Optics and Catoptrics give a good idea of the state of knowledge 

 of the subject at the time when Euclid lived, 330-275 b.c, and 

 that the subject of the refraction of light had not been studied 

 then. 



It is from the Optics of Claudius Ptolemy, the astronomer of 

 Alexandria, who flourished at the end of the first century, that 

 we obtain our first data on refraction. This work, which 

 consists of five books translated from the Arabic into Latin, 

 was not printed until 1885. It was known in the time of Roger 

 Bacon, but escaped notice until 1816, when Delambre published 

 an analysis of it from the manuscript in the Royal Library at 

 Paris. A good account of it is given by Sir David Brewster 

 in the article on Optics in the eighth edition of the Encyclopedia 

 Britannica. 



The fifth book deals with refraction. First of all, there is 

 an explanation of the experiment with the coin and the basin 

 of water, which attracted a considerable amount of attention 



