256 Dr. Goring"'s Commentary on ; 



object, and a stop in its focus to determine the aperture, with 

 another piano lens immediately behind the said stop, the plane 

 side of which glass should also be next the radiant *, the whole 

 being on the principle of the erecting eye-piece of an achromatic 

 telescope — such object-glasses are neither expensive nor difficult 

 to make, and I think might be adopted by such as cannot get 

 or make aplanatics. Most of the easier transparent test-objects 

 are shewn by the old compounds, provided we use an object- 

 glass, which has by itself the power of shewing them as a sim- 

 ple microscope — such as a lens of J, |^, -^q, -^, or J^ of an inch 

 focus, as / have frequently remarked in my writings. The 

 only consequence is, that when the power of such lenses has been 

 quadrupled by the action of the compound body attached to 

 them, the object is seen infinitely worse than by the simple 

 glass — still, however, it is seen — and for many individuals this 

 is quite enough f. It is astonishing how coarse an organ the 

 eye is in the majority of mankind, and how easy it is to de- 

 ceive them in such things as microscopes. I would recommend 

 every man who wishes to see any microscope as it ought to be 

 seen, to go to the maker of it — he will be sure to make it put 

 forth its mettle — for it is to be observed, that there is but one 

 way of shewing a microscope as it ought to be seen, hut fifty 

 others — nothing is so easy as to make a good instrument appear 

 a bad one, by injudicious or knavish management. 



* The forms here recommended are good enough, but not the hest which might 

 be employed : the fact is, that the apertures ■wh.ichobject lenses and doublets of the 

 most perfect figures will bear without exhibiting so much chromatic aberration as 

 would destroy all distinctness, is so very inconsiderable, that it is not worth while to 

 correct the spherical aberration to any great extent, for it produces no sensible be- 

 nefit. Before I caused the achromatic object-glasses to be made, I h-ied, I think, 

 every possible construction to procure achromatism without them, but so strictly do 

 engy scopes follow the law of telescopes, that unless both the objective and ocular 

 parts are achromatic in themselves, the whole will not be so — if two Huygenian eye- 

 pieces are employed, one for the object-glass, and the other for the eye-glass here 

 will be the double compensation, and they will make an achromatic but very indis- 

 tinct erecting eye-piece for a telescope : it cannot, however, be used as a microscope 

 upon a real object, because the focus is between the two objective glasses ; nor can this 

 defect be altered or distinctness attained mthout destroying the achromatism. 



f To shew how completely achromatic glasses are thrown away on some, I 

 shall give the following anecdote : — A certain worthy personage had a very good 

 achromatic object-glass, with achromatic Huygenian eye-pieces : he was not content 

 with the field of view given by these, but got an eye-piece made of four or five 

 glasses on the good old plan, to give a swinging large expanse of field — of 

 course this complicated composition would not be rendered achromatic; — what was 

 to be done next ? why the object-glass — which wanted no reduction of its aperture, 

 was cut off; till the faintncss of its light rendered the aberration of the eye-piece 

 less sensible— the whole was considered a vast improvement ! 



