• Dr. Goring on Achromatic Microscopes. 421 



gnment sake, (it must be recollected I do not assert the fact,) 

 that the three instruments showed some difficult proof-object 

 equally to the satisfaction of the same observer ; is this to show 

 that the three instruments are equally good ? Most assuredly 

 not; quite the reverse. The experiment would demonstrate 

 that the principle of the instrument which had the objective 

 part of the least natural penetrating power, was decidedly 

 superior in doing as much with 0*9 focus as the achromatic 

 with 0*4, and the common compound with O'l. In order to 

 give to each its true position with regard to the others, let 

 the objective part in each be rendered of the same focal dis- 

 tance, of the same angle of aperture ; and the relative capa- 

 bilities of the three instruments must then necessarily be de- 

 veloped. As Amician metals and achromatic object-glasses 

 cannot well be made of 0- 1 focus, let the common object- 

 glass be elongated to 0*9, and the achromatic also ; then com- 

 pare them together, and a glance will suffice to show them 

 in their true colours. 



In fact, nearly all the test objects are just visible with a 

 single lens of -j^th of an inch focus and the requisite aperture. 

 It is, therefore, not very meritorious in the common com- 

 pound to do as much as its object-glass will by itself, con- 

 sidering that the power gained by attaching the body to it 

 will triple what it was in its natural, unassisted state. The 

 excellency of the other instruments consists in their being 

 able, when only equal in power to the said single 0* 1 of an 

 inch, to equal and indeed surpass it in point of performance. 

 As to what a common compound will do with the same 

 power obtained from an object-glass of 0*9 focus, and 0*45 

 of aperture, I think I need not mention, for its image will 

 be a complete mess of aberration of both kinds. 



The common instruments act much better^ in proportion, 

 with very deep object-glasses, having large angles of aper- 

 ture, than with shallow ones in the same predicament ; 

 because the diameters of the former will necessarily be ex- 

 ceedingly small, and therefore admit so small a quantity of 

 light, that however violent their aberration may be, but 

 little becomes sensible, for as colour is nothing but de- 

 composed light, where there is but little to be decomposed, 



