THE PRESENT CONDITION 0¥ ENGLISH OBJECT-GLASSES. 261 



expressed, ' Monthly Micro, Journal,' p. 302, 1870, in reply- 

 to the researches of the writer, on the residuary aberration 

 of the microscope : — 



" Objectives from the hands of careful and experienced 

 makers have all been constructed on the globule test, and 

 are not sent forth till every error of workmanship, centering 

 state of oblique pencils, achromatism, and S2:)herical aberra- 

 tion, are all absolutely corrected (sic, except the italics) ; 

 for this discovers the least fault of either when all others fail." 



Those who now enjoy the beautiful definition of first-class 

 objectives remember with regret the innumerable labours 

 they have thrown away in trying to see nature through a 

 bad pair of spectacles, which often misrepresented and cari- 

 catured 'some precious object, and displayed nothing but 

 false images of real structure. Curiously by one of the first 

 axioms of Euclid, the Egyi:)tian, a line was defined 300 

 B.C. to be an assemblage of points ; but the microscope, 

 which for a hundred years had shown lines until recently in 

 many objects, failed to show the points of which the line was 

 composed. Accordingly, the word " lined-object" has dis- 

 appeared from the new nomenclature of the microscopist. 

 When defining power was in its infancy, a wonderful dia- 

 mond or sapphire doublet was prized for the stria it could 

 develop. I have in my possession a doublet, of Pritchard's 

 make, of ^th of an inch focal length, of very small angular 

 aperture, which shows the Podura exclamation-markings 

 on a test-scale {P . curvicollis) . 



This doublet, used as a condenser, develops a remarkably 

 clear, pure, and beautiful miniature image of a brilliant 

 flame. In the case containing this valuable relic of the 

 past, there are about sixteen others, the defining power of 

 which seems exactly measured by the precision of the bril- 

 liant image miniatured by them when examined by a first- 

 rate microscope. 



Now, if for this doublet a fine -^th immersion be sub- 

 stituted, and the microscope be armed also with a fine ^th or 

 -'^th immersion lens, so that the miniature be formed within 

 the substance of the drop of immersion fluid, using a low 

 eyepiece, all the deviations of the miniature, from what 

 we know ought to be the appearance of the miniature, can at 

 once be determined. 



A great variety of effects will be discovered by slightly 

 altering the screw collars of the miniature forming- and 

 observing-object-glasses. A small brilliant flame, or glitter- 

 ing object, or a distant table lit up by daylight or sunbeams, 

 the microscope being placed horizontally, so as to form the 



