418 Dr. Goring on Achromatic Microscopes. 



provements on the microscope (which must surely form a 

 new epoch in the history of that instrument) may be made 

 to harmonize and blend together in one omnipotent whole, 

 wherein the defects and incapacity of each, for particular 

 purposes, may be borne out and neutralized by the pecu- 

 liar power and distinct operation of the other component 

 parts ; for I conceive it impossible to find any microscopic 

 objects, capable of being developed to human vision, which 

 one or other of the three microscopes will not master and 

 exhibit in the utmost possible perfection. 



Test Objects. 



Without test objects it is impossible to ascertain whether 

 a microscope is effective or not : few opticians have any 

 other ideas concerning the goodness of microscopes than that 

 they must have a large flat field of view ; presuming that in 

 proportion to the width of the field-bar and the flatness of 

 the image at the margin of it, must the perfection of the in- 

 strument be estimated, without any regard I suppose to light, 

 distinctness or achromatism ! Nothing can be more absurd 

 and preposterous than this notion, with which, however, I 

 was at one time of day as much imbued myself as any worthy 

 shoptician or glazier can be at this moment. It is just as 

 ridiculous to estimate the goodness of a telescope in the same 

 way. 



I willingly admit that a large flat field of view is a valua- 

 ble property in either instrument ; but, after all, if we can 

 see nothing which ought to be seen in it, it is of no more 

 value than a large piece of ground which will produce no- 

 thing but weeds and thistles. The prevalence of these false 

 ideas must be attributed to the want of the knowledge of 

 proper objects to try the goodness of microscopes in the same 

 way as that of telescopes. I believe I must appropriate to 

 myself the honour of having first discovered such, of which 

 I now propose to give some account. A test object, then, 

 may be defined to be some object xohich can only be seen 

 by a microscope single or compound^ which possesses a great 

 quantity of good distinct light (the result of a large and per- 

 fect aperture)^ and which in consequence will serve as a 



