Dr. Goring on Achromatic Microscopes. 419 



standard of comparison, whereby to scrutinize the preten- 

 sions, the defects, and excellencies of different microscopes, 

 and prove beyond dispute whether they are effective or not*. 

 They will serve as a capital subject of study for those who 

 suppose the goodness of a microscope to depend upon the 

 width and flatness of the field of view. 



I here premise a circumstance which must be rigidly at- 

 tended to, in order to give an instrument fair play : it is that 

 the power of any two microscopes, tried against each other, 

 should be equalized and obtained in the same way ; for the 

 better an instrument is, the lower will be the power necessary 

 to give it the ability to demonstrate a difficult object. That 

 microscope, therefore, which will show given objects with the 

 lowest power is most certainly the best, be it what it may. 

 The analogy holds good with regard to telescopes also ; for 

 I have ever remarked, that all the perfect instruments of 

 this kind which I have seen were distinguished by their 

 power of showing difficult double-stars, &c., with very low 

 charges. Nothing exasperates me more than to hear 

 some half-taught booby bragging of the goodness of a mi- 

 croscope because it shows objects so wonderfully magnified, 

 when, in all probability, it will be found completely inert 

 and ineffective when called upon to show them with a reason- 

 able working power: thus converting what is really a proof 

 of its worthlessness into an argument for its supposed supe- 

 riority; for I myself shall ever maintain high magnifying 

 power to be an evil, and that the less we can use of it the 

 better, for it is sure to diminish the quantity of the object 

 within the angle of visible field ; to attenuate and dilute light 

 and distinctness ; to prevent us from having more than a 

 point of an object truly in focus at once ; and, finally, to ren- 

 der our instrument tremulous and totally unmanageable, to 

 say nothing of the injury done to the observers' eyes by the 

 strain they must suffer from difficult vision. 



In order to ascertain the equality of power, there is no 

 method more sure or effectual than to employ the dynameter 



♦ I call a compound microscope effective when it shows everything 

 which can be seen with single-lenses power for power, without the co- 

 loured fringes and fog of the latter. 



