ARABIAN AND MEDIEVAL SCIENCE. 67 



tical mechanics known in his time. His attainments in astronomy 

 were far superior to those of his contemporaries. His inventions provfe f 

 his skill in practical mechanics and in chemistry, and many of the 

 schemes he proposed have since been realized. Thrc/?omposition of 

 gunpowder was known to him, although he does not give y /be ^details 

 of its preparation practically. The principle of the diving-bell' is <cj/ear.ly 

 described in his works, and many automatic machines were projected 

 or constructed by him. It is said that he constructed a brazen head 

 which would speak and answer questions addressed to it. This is an 

 acoustical curiosity easy enough to understand now, but in Friar Bacon's 

 time it was naturally regarded as proof of the magical power of its 

 fabricator. 



Bacon wrote many treatises, and the chief of these are comprised in 

 his book, first published, under the title of " Opus Majus" in 1266, 

 and first printed in London in 1733. This work is remarkable for 

 the great width of its views and well-grounded observations on various 

 optical subjects, such as astronomical refraction, the apparent size of 

 objects, and the peculiar appearance of the sun and moon when upon 

 the horizon. These subjects were treated, as the reader is already 

 aware, by Alhazen, and his works are quoted by Bacon, as is also a 

 treatise by Ptolemy on optics which is not now extant. Bacon dis- 

 cusses various optical questions, such as the foci of spherical mirrors, 

 the round image formed by rays of the sun passing through a small 

 opening of any shape whatever, the nature of vision, etc. but although 

 in some cases he seems to be approaching the true explanation, he 

 usually falls short of it. Much discussion has arisen on the optical 

 discoveries of Roger Bacon, some having claimed for him the merit 

 of being the inventor of the telescope, while others deny that he had 

 any practical acquaintance with even the simple lens. He does, indeed 

 mention that a segment of a sphere of glass would magnify letters on 

 which it might be placed with the plane side downwards, so as to have 

 its convexity towards the eye. This, however, is only what Alhazen 

 had previously stated in his work on optics, observing also that the 

 larger the segment of the sphere the greater the magnification. Bacon, 

 on the other hand, expressly states that the smaller the spherical seg- 

 ment, the better will the letters be seen and the larger will they appear. 

 This being contrary to fact, would seem to prove that Bacon could 

 not have made the trial ; and when, immediately after the passage here 

 referred to, he states that a flat piece of glass will, produce the same 

 effect, it becomes more difficult to believe that he had made any ex- 

 periments on these effects. The passage which has been interpreted 

 as referring to the telescope does not in reality specially describe such 

 an invention, but it is interesting for its foresight of the effects which 

 have since been realized by such optical instruments as the telescope, 

 the microscope, the multiplying-glass, etc., all depending upon the 

 principle of refraction. " We can so shape transparent Bodies, and so 



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