Dr. Goring on the new Microscopes^ Sfc. 109 



very small. It is curious to see how the aberration of a 

 single object-glass varies with different angles of apertures : a 

 good triple 0.9 focus will, with an aperture of only 0.2, incline 

 to have its spherical aberration in the convex, with 0.35 it will 

 be about correct with 0.4, the spherical aberration will incline 

 towards the concave, and with 0.5 it will be very overpowering 

 in the concave. 



If two are combined, which both work well in a detached 

 state, the sa«ie state of aberration still continues, though with 

 diminished effect. It is only when the anterior object-glass is 

 made expressly to correct the other, and to be useless by itself, 

 that we get an approximation to that perfection in figure 

 which it is always practicable but difficult to give to metallic 

 surfaces. 



" A metal of 0.6 focus with 0.3 of aperture, or any other 

 focus with the same angular opening, I have frequently seen 

 so figured as to be quite perfect, showing, when put out of focus, 

 no tendency either to sphericity or parabolism, but a true and 

 genuine elliptic curve, giving with a low power that intensiva of 

 distinctness which in my opinion will never be obtained from any 

 refracting instrument, however excellent. Simplicity must, I 

 think, ever be held to be a capital ingredient in the perfection 

 of works of art ; and nothing can be more simple than the 

 operation of a reflecting instrument, or more complicated than 

 those of a thick sextuple or double triple aplanatic object-glass 

 of large aperture, for diverging rays : there is perhaps not a 

 mathematician in Europe who dares to face the theory of it. 

 It has always been an admitted fact, that reflecting telescopes 

 would beat refractors, was it not for their want of light ; but it 

 has been shown, I think, that in reflecting microscopes this 

 defect is of little or no practical consequence. There is an 

 object which I have never yet seen satisfactorily with a refrac- 

 tor, viz.y the cross striae on the feathers of the Papilio Brassicse 

 and some others of that class ; whereas a reflector of 0.3 focus 

 and 0.2 of apertures brings them completely out either by day- 

 light or lamp-light in their full complement, and by the former 

 allumination so as to be visible along with the longitudinal 

 lines, presenting the appearance of a piece of brick-work. 



The radiant point in the Amician microscope approximates 



