of the Microscope. 425 



Again, in a telescope, the portion of the image used is sensibly 

 flat, though the radius of curvature of every such imao-e, is 

 about |ths of the focal length of the object-glass*: but in the 

 microscope it is evidently far otherwise, so that were the whole 

 image distinct, it would still be impossible to have any great 

 extent of it distinctly visible at once ; and this objection applies 

 in full force to the most perfect achromatic object-glass. Now 

 with a sphere, properly cut away at the center so as to reduce 

 the aberration, and dispersion, to insensible quantities, which 

 may be done most completelyf and most easily, as I have 

 found in practice, the whole image is perfectly distinct, what- 

 ever extent of it be taken, and the radius of curvature of it is 

 no less than the focal length, so that the one difficulty is 

 entirely removed, and the other at least diminished to one 

 half. 



Besides all this, another advantage appears in practice to 

 attend this construction, which I did not anticipate, and for 

 which I cannot now at all account. I have statedj that when 

 a pencil of rays is admitted into the eye, which, having passed 

 without deviation through a lens, is bent by the eye, the vision 

 is never free from the coloured fringes produced by excentrical dis- 

 persion. Now with the sphere I certainly do not perceive this 

 defect, and I therefore conceive that if it were possible to make 

 the spherical glass on a very minute scale, it would be the 

 most perfect simple microscope, except perhaps Dr. Wollaston's 



• I suppose the lens to be of glass. In any case it is less than half the focal 

 length. 



+ The only limit in practice to the diminution of the aperture, is the danger of 

 the glass breaking, for the loss of light is trifling, in comparison of that which is un- 

 avoidable in many other constructions. 



I Treatise on the Eye and Optical Instruments, Art. 325. 



3l2 



