266 Dr. Goring's Commentary on 



their merit may be, which are not in contact, as unfit for mi- 

 croscopic purposes, for it must be evident, that the greater the 

 distance between the lenses is, the nearer will the external 

 focus approximate to the surface of the glass next the object, 

 so that before the combination arrives at any considerable 

 power, it is impossible to view opaque objects with it, and a 

 little further on in the scale, it becomes impracticable to use it 

 on transparent ones also : thus Dr. Wollaston's combination, 

 before it reaches at the power of one-twentieth of an inch, is 

 unable to view the objects in a common microscopic slider, 

 having its inferior side where there are no rings, presented to 

 it : in fact, it can only look through the thickness of a piece of 

 talc ; and I have before observed, that it is necessary for simple 

 microscopes greatly to exceed the power of one-twentieth of 

 an inch, to enable them really to beat the new engyscopes. 



Mr. Herschel, in a memorable paper in the Transactions of 

 the Royal Society for 1821, has been so kind as to favour us 

 with the best possible way of making microscopic doublets 

 both with plano-convex and other lenses. He selects two 

 plano-convex lenses, having their foci in the ratio of 1 to 

 2 . 3, a proportion not very different from Wollaston's, but 

 he does not place them with an interval of 1 . 3 between 

 them, (by which he would only have reduced the aber- 

 ration to one quarter of that of an equi-convex,) but in 

 contact with their convex surfaces towards each other, the 

 weaker lens being next the eye — having discovered that by so 

 doing he reduces the aberration to less than one quarter of 

 that of a crossed lens of the least possible aberration, or 0.2481, 

 and diminishes the distortion and prismatic dispersion into the 

 bargain ! His combination could also be executed cheaper 

 than Wollaston's, for there would be no trouble in measuring 

 the distance between the lenses. Not content with this, 

 Mr. Herschel has given us two other combinations of crossed 

 lenses, with menisci, which destroy the spherical aberration 

 in the axis altogether ! What can we have more ? Is it 

 not too bad to go on experimenting, as if this colossus of 

 science had d^ne nothing ? I cannot conceive it possible that 

 any person taking up the subject after Mr. Herschel, can pro- 

 duce better combinations, the subject is therefore exhausted. 

 It would surely be the most insufferable presumption in any 



