and on the Memoir of Eulrr. 247 



Ratlins of 3d surface, 0.5 ) ,,,,. . 



4th 5 0.5 j concave > (Flint) 



5th, 0.575 ) m , u , t , 



6th, 0.575 } convex > (Dutch Plate) 



Specific gravity of crown, 2.527 \ 



Thickness of do. 0.15 j 



Specific gravity of flint, 3.627 | 



Thickness of do. 0.1 (vi j 



Specific gravity of Dutch, 2.519 \ 



Thickness of do. 0.175 j 



Any mathematician may see that there is in this object-glass 

 a vast excess of aberration in the convex lenses, beyond what 

 would be required in a telescope, which ever way the rays pass 

 through the lenses. 



I once procured the object-glass of a small perspective of 

 little more than four inches focus, and somewhat more than 

 an inch of aperture, and converted it into the object-glass of a 

 long engyscope by reversing it. I regret that I did not make 

 notes of my observations with it, as I cannot now obtain it 

 again ; I only recollect that it did not answer at all, and was 

 not even achromatic used in this way. If I can rely on my 

 memory, this object-glass, used in its natural way as a tele- 

 scope, had an excess of aberration in its convex lenses, (at least 

 with its whole aperture,) — a circumstance which should have 

 operated much in its favour. 



I conceive, therefore, that Euler has done little or nothing 

 beyond pointing out the application of achromatic glasses to 

 form the images of engyscopes, both because he has selected 

 an angle of aperture much too small for efficiency even on ob- 

 jects not of lined hind, and because his system of reversing the 

 order of the curves of an achromatic, in order to adapt it to 

 diverging rays, will be found utterly inadequate to the pur- 

 pose. The affair will not be got over .so easily I am sure, but 

 will prove a confounded tough job for the proudest mathema- 

 tician in Europe. The talents of Euler were perhaps little 

 inferior to those of any man living or dead. Had he taken up 

 the subject in the right point of view he would doubtless have 

 succeeded. 



A double object-glass agrees rather better with his theory 



