52 THE EARLIEST STEPS IN THE INVENTION OF 



ideas on the structure of the eye that wero an improvement on 

 those of his predecessors, but he had little knowledge of lenses, except 

 in connection with that organ. He does, however, refer to the 

 magnifying power of segments of a glass sphere. He considered that 

 vision resulted from rays coming to the eye from the object, and 

 opposed the view, which held the tield till the seventeenth qentury and 

 later, that explained vision as a result of something emanating from 

 the eye. There are editions of Alhazen's work printed in the six- 

 teenth century. These represent a translation into Latin by an 

 unknown writer of the late twelfth or early thirteenth century 

 (sec 4). 



(4) Witchj (first half of the thirteenth century) was a Pole, who 

 studied in great detail the work of Alhazen. His own work 

 grew out of this, and is perhaps an improvement on it. Thus 

 he drew up a table of refractions for the three media — air, water, 

 and glass — from which it could be seen that the angle of refraction 

 did not vary according to the angle of incidence. It is doubtful, 

 however, to what extent these tables were original or the results of 

 direct observation. The works of Alhazen and of Witelo were printed 

 together by F. Risner at Bale, 1572. An interesting account of 

 Witelo, together with a reprint of his Perspectiva from the MSS. 

 has been recently s,e.t forth by Clemens Bauemker in his Beitnuje 

 zur Geschichte der Philosopliie des Mittelalters, Munich, 1908. 



(5) Roger Bacon (1214-1294) accomplished real advances in the 

 knowledge of optics. His work was based primarily on Latin trans- 

 lations of Arabian writers, and especially on Witelo's version of 

 Alhazen. He is distinguished from his predecessors, however, by 

 his clear conception of the value of experiment, and by the evidence 

 in his works that, having made a serious and continuous effort to 

 discover the laws of the refraction and reflection, he sought to apply 

 his knowledge to the improvement of the power of vision. In this 

 he is a real pioneer, and is in the truest sense the father of micro- 

 scopy. 



But it is easy to exaggerate the claims of Bacon, and the wildest 

 statements are often made about his discoveries. It is a fact that 

 there is no evidence that he ever made a telescope nor any micro- 

 scope, save a simple one. But he had a clear, though not wholly 

 accurate idea of the nature and properties of lenses, and, groping 

 with the instinct of genius, he did vaguely foresee both telescope 

 and microscope. The following passages will serve to indicate the 

 stage he had reached in optical knowledge. I have purposely 

 selected passages containing some errors. It will be observed that 

 in the first of these passages Bacon refers to and figures the object 

 as though it were itself in the denser medium of which the lens is 

 composed. In doing this he is confusing the optical action of a 

 lens with that of a liquid in which an object is immersed. The 

 optical results of immersion in a liquid had been investigated by 

 his predecessors, and were perhaps familiar to Aristotle. 



If anyone examines letters and other minute objects through 

 the medium of crystal or glass or other transparent substance, if 

 it be shaped like the lesser segment of a sphere, with the convex 

 side towards the eye, and the eye being in the air, he will see the 



