Vision and Optical Glasses. 189 



truly illustrious countryman Roger Bacon, who 

 demonstrated by experiment that a small segment 

 of a glass globe would assist the sight of old 

 persons. Thus he may be regarded as the person 

 who first discovered the theory of spectacles, 

 though they were not brought into use until the 

 following century. 



The telescope was invented about the end 

 of the sixteenth century, and the discovery is 

 commonly supposed to have been casual. The 

 account which is generally received is, that the 

 children of Zacharias Jansen, a spectacle-maker 

 of Magdeburgh, trying the effect of a convex and 

 concave glass united, found that when placed at 

 a certain distance from each other, they had the 

 property of making distant objects appear nearer 

 to the eye ; but the reason of this effect was not 

 discovered till the time of Kepler. 



The microscope was also an invention of Jansen 

 or his children: and as it is rather a simpler 

 instrument than the telescope, it will serve to 

 introduce you very properly to a knowledge of 

 these kinds of glasses. You already know that 

 the nearer any body is to the eye, the larger is 

 the angle under which it will be seen ; but if 

 placed too near, the image will be confused, 

 because the divergence of the rays is then too 

 great to admit of their being properly converged 

 on the retina by the humours of the eye. In fact, 

 an eye which is not near sighted cannot discern 

 any object clearly at a shorter distance than six 



