M. Euler on the Perfection of the Microscope. 235 



sides, having its focal distance 0,229, and therefore the radius 

 of each of the two sides 0,274. 



4. The third lens of croum-glass must be equally convex on 

 both sides, so that its focal distance may be 0,375. We must 

 give to the radius of the front side 0,644 of an inch, of the 

 other side 0,287. 



5. We must combine these three lenses, so that the dis- 

 tance between the middle of the second lens and that of the 

 first or third, should be only 0,019, so that the thickness of 

 the whole object-glass will be about 0,057. 



0. The object which is to be examined must be always 

 placed at the distance of half an inch from the object-glass ; 

 and then there will be no reason to fear that the too oreat 



O 



proximity of the object may cause the least inconvenience. 

 On the contrary, we shall see all the different parts of the 

 object nearly with the same degree of distinctness. 



Description of the two Eye-Glasses. 



7. It will be proper to form them both of flint-glass, for the 

 purpose of admitting a greater aperture, which will contri- 

 bute greatly to increase the apparent field, making both of 

 these glasses equally convex on both sides. 



8. The first of these eye-glasses, the one which faces the 

 object-glass, must have the radius of each side 1,200, and the 

 diameter of its aperture 0,600. 



9- The second eye-glass, turned towards the eye, must 

 have its focal distance 0,333, and the radius of each side 0,400, 

 and the diameter of the aperture 0,200. 



10. Beyond this glass we must place the eye at the distance 

 of 0,167 of an inch. 



11. The distance between these two eye-glasses will be al- 

 ways |ds of an inch, as, according to the nature of the eye, the 

 last eye-glass must be sometimes nearer and sometimes farther 

 from the first, which is effected very easily by means of a screw. 



Having made these three glasses, the magnifying power 

 will depend only on the distance between the object-glass 

 and the first eye-glass, to which it is always proportional. 

 But we must remark, that the more we increase' t iu- magnify. 



