428 fflSTORY OF LENSES AND MICROSCOPES [Ch. XII 



§ 693. Cylindrical spectacles — astigmatism. — The use of specta- 

 cles for defective vision was, until quite recently, confined to those 

 with long sight or short sight, and spherical convex or concave lenses 

 were used. Two English astronomers and physicists (Thomas Young, 

 1800, and George B. Airy, 1825) found that the curvature in their 

 eyes were not equal and consequently lines in one plane focused at 

 one level and in another plane at a different level. To correct this 

 defect Young pointed out that the spectacles might be tilted, and 

 Airy that the best way was to use cyUndrical glasses which would 

 just neutrahze the unequal curvature. Probably this discovery of 

 astigmatism and the means for its correction has and is destined to 

 accompUsh greater good to the human race than any other optical 

 device of the 19th century. 



Some little space has been devoted to spectacles because the eyes 

 are a fundamental part of any optical combination like the microscope 

 or telescope and is the judge of the real images produced by any optical 

 train like the photographic camera, the magic lantern and the mov- 

 ing picture machine, therefore whatever pertains to the eye and its 

 natural perfection or artificial means of making it more perfect, is 

 germane to the subject. 



§ 694. Simple microscope. — Every convex lens is or may be used 

 as a microscope, as it aids the eye in seeing an object under an in- 

 creased visual angle, and hence makes it appear larger than it would 

 if viewed by the naked eye. Hence, when considering the history of 

 the simple microscope it is evident that that history is the same as 

 the history of convex lenses. The date of the invention is some time 

 before the date of the Opus Majus of Roger Bacon. He speaks of 

 them, not as a wholly new invention of his own time, but as one 

 of the means by which wonderful things can be done. His whole 

 purpose in the discussion was to induce the church to make the 

 fuUest use of all the products of science to give the superiority which 

 he felt was the right and the privilege of the Christian world to 

 possess in its efforts for advancing civilization. 



The simple lens or the combination of lenses making up a simple 

 microscope may be held in the hand, but ordinarily there is some 

 metal binding and support for the protection of the lens or lenses, and 



