2*12 Dr. Goring on the new Microscopes, Sfci 



, I have thus (though I am afraid in a very ill arranged and 

 rambling way) stated the different essential virtues and capaci- 

 ties of refractors and reflectors, according to my own ideas ; 

 it remains for me to compare the compound instruments with 

 the single ones, and the subject will be exhausted: this I have, 

 in fact, in a great degree, done in a note on Mr. A. Pritchard's 

 paper on diamond lenses ; so that it will not be requisite for 

 me to give more than a slight recapitulation of what I have 

 there advanced, with a few remarks on the action of the apla- 

 natics used as single microscopes. 



The aberration of ordinary lenses is not thoroughly displayed 

 until they are caused to form a picture ; when we merely use 

 them as a medium to enable us to view objects under large 

 angles, they operate reasonably well, never producing indistinct- 

 ness enough to preclude the vision of anything they ought to 

 show, though they may veil it in a disagreeable vapour of scat- 

 tered light, and fringe it with prismatic tints. On this account 

 there is not a difference between the performance of achromatics 

 and common lenses, used as single microscopes, proportional to 

 that which they exhibit as object lenses. On a former occasion 

 I have remarked, that the figure of an aplanatic intended to be 

 employed as an object glass, is different from what is required 

 for a mere magnifier. To the latter purpose, however, their 

 curves might be adapted, and we should then arrive at the 

 extreme intensity of distinctness in viewing a real object. As 

 however usually made, if a sufficient reduction of their aper- 

 ture is effected, their performance is faultless. Nothing can be 

 more beautiful than the action of an achromatic of about an 

 inch focus used with any aperture (for the iris is sure to reduce 

 it sufficiently); the shorter foci of course must be cut off, other- 

 wise their false marginal rays will be able to clear the opening 

 in the curtain of the eye — I do not think we shall ever have 

 any single achromatics of shorter focis than about 0.2 inch, a 

 power which is capable of doing a vast deal of useful labour, 

 as indeed an inch focus will also. 



What we want in microscopes, and what we should boast of, 

 is to he able to see everything with very low powers. It is as- 

 tonishing when we really go to work with good microscopes at 

 investigating nature, how seldom it is necessary to use a high 



