Dr. WollaBiovLS Microscopic Doublet 267 



experimentalist to expect to arrive at such curves. If it is ad- 

 mitted that he might, then may analysis be considered mere 

 useless subtlety and refinement, like the antique practice of 

 shoeing horses by witchcraft. Some years ago, I caused Mr. 

 Herschel's combination, Fig. 5, to be executed to a focus of one- 

 sijsth of an inch, which I employed as the eye-piece of a seven 

 foot Newtonian telescope ; but, as the power it gave was consi- 

 derable, and the adjustments of my instrument were not good, 

 I could not keep the object truly in the axis of the lenses where 

 the main distinctness lies ; therefore, I did not feel the peculiar 

 efficacy of the combination, so I made a microscope of it, and 

 in this way it shewed very great penetrating power, for I saw 

 with it many lined objects which I never saw loith so low a 

 power before: 1 tried it against a triple aplanatic oi one-fifth of 

 an inch focus, and hoped that the latter, having its spherical 

 aberration completely destroyed, and its chromatic corrected 

 also, would be able to shew the objects exhibited by Mr. Her- 

 schel's doublet, but found, very much against my will, that it 

 would not. Now, I conceive, and such is the result of my expe- 

 rience, that there is a fair way of proving microscopes by trying 

 which can shew a given object with the lowest power, for very 

 great distinctness will, to a pertain extent^ compensate for mag- 

 nifying power. 



, Thus, suppose some telescope gives a remarkably small 

 round image of a star, free from all irradiation, might it not 

 be proved mathematically, that it must separate a double one 

 with a lower power than another telescope which gives a large 

 irregular image confused by aberrant light, whereby the images 

 of the two stars are so blended together, so that it must require 

 a high power to force them apart, if indeed it can dp more 

 than shew the star elongated ? Now, what close double stars 

 are in a telescope, the lined objects are in a microscope, for 

 they can be only seen with the low powers^ when their image 

 is uncommonly sharp, fine, and distinct, and the space between 

 them clear and transparent. I shall here give a slight sketch 

 of the best sorts of simple microscopes : first, come the 

 aplanatics, in which both the chromatic and s[)herical aberra- 

 tion is destroyed by concaves of flint-glass *. These, however, 



* As the eye-piece of a telescope is nothing but a microscope for examining its 



