231 M. Euler on the Perfection of the Microscope. 



have only half an inch of focal distance, and which could re- 

 ceive an aperture of the eighth-part of an inch in diameter. 

 This would furnish for all the magnifying powers a sufficient 

 degree of distinctness ; but as such object-glasses will cause 

 no confusion either from the aperture or the different refrac- 

 tions of the rays, the greatest advantage, without doubt, will 

 be that we could see all objects with the greatest clearness and 

 distinctness, which will enable the natural philosopher to bring 

 microscopic observations to the highest degree of perfection. 



To facilitate the more the execution as well as the use of 

 such microscopes, we shall give here the description of one, 

 which contains only two eye-glasses, and which might serve even 

 to produce all the magnifying powers, from the smallest to the 

 greatest, without being obliged to change any thing, either in 

 the object-glass, the eye-glass, or in the place of the object, 

 which should be put always at the distance of half an inch be- 

 fore the object-glass. The only variation is to be produced 

 in the distance between the object-glass and the first eye-glass, 

 which will be removed in proportion as we wish to magnify 

 the object. 



Description of the Object-Glass. 



1. The object-glass should be composed of three lenses, of 

 which the first and third are made of crown-glass, and the se- 

 cond of that sort of flint-glass whose index of refraction is 

 1,600; so that the focal distance of the whole object-glass 

 may be half an inch, and that it may receive an aperture of 

 the one-eighth of an inch in diameter. For this purpose the 

 three lenses of which this object-glass is composed, will repre- 

 sent little disks, whose diameter will be about the one-seventh 

 of an inch, and they must be made as thin as their figure will 

 allow. For the figure of each we shall give here the measures 

 expressed in thousands of an inch of twenty-seven milli- 

 metres. 



2. The first of the three lenses, the one which is turned to- 

 wards the object-glass, must be of crown-glass, equally con- 

 vex on both sides, so that the focal distance will be 0,28b', 

 and therefore the radius of each of its sides 0,301. 



3. The second lens of Hint-glass, equally concave on both 



