Ch. XII] HISTORY OF LENSES AND MICROSCOPES 431 



The demonstration of the principles on which such a mi< r<»< opr or 

 telescope could be constructed is to be found in the Dioptrica of Kepler, 

 Proposition LXXXVI. The proposition is: With two convex Lenses 

 to show objects larger and inverted. 



In Prop. LXXXIX, it is stated that with three convex lenses 

 can be shown objects enlarged and erect. This is the principle 

 of the terrestrial or erecting telescope. 



Kepler first showed the real action of the eye as an optical instru- 

 ment, and that the retinal image must be inverted, and that unless 

 inverted, objects would appear wrong side up. Now we know that is 

 true, for it is an easy demonstration to show, as did Scheiner in 1619- 

 1625, that the retinal image is actually inverted in the eye of an 

 animal or man. 



As Kepler showed the actual dioptrics of the eye, he was the 

 first to explain the real action of spectacles in correcting the 

 defects of long sight and short sight, viz. to aid the refracting 

 surfaces of the eye to make a sharp image of the object upon the 

 retina. 



While Kepler gave the optical demonstration for a microscope or 

 telescope with convex lenses, he, so far as known, did not actually 

 construct such a microscope or telescope. Christopher Scheiner, 

 while he lacked the original genius of Kepler for discovering and ex- 

 pounding principles, had greater mechanical ability. He actually 

 constructed the Keplerian telescope and microscope and used them 

 both for observation and for projecting real images. On page 130 of 

 the Rosa Ursinae (1626-1630) occurs this remarkable passage: "In 

 the same way [i.e. by two convex lenses] was produced that wonderful 

 microscope by which a fly was made as large as an elephant and a flea 

 to the size of a camel." 



§697. Binocular microscopes. — From the first invention of the 

 telescope-microscope there was dissatisfaction that it was for but one 

 eye, and before 1610 there were made those for both eyes by putting 

 two equal instruments side by side the right distance apart for the 

 eyes of the observer. That arrangement of the Dutch telescope still 

 holds in opera glasses. 



One of the first examples shown in pictured form is that of the 



