EAKLY CONDENSERS 299 



' exclamation ' marks, and as this is the experience of the majority 

 of efficient experts, it may be taken that no resolution of these was 

 accomplished in pre-achromatic days ; these lenses, in fact, over- 

 lapped the discovery of Achromatism. 



But the practical results of the use of achromatic lenses soon led 

 experienced men, understanding their theory and practice, to perceive 

 that if it were good for the lenses which formed the image, it was 

 also good for the condenser. Thus Sir David Brewster in 1831 ad- 

 vocated an achromatic condenser in these remarkable words, viz. : ' I 

 have no hesitation in saying that the apparatus for illumination 

 requires to be as perfect as the apparatus for vision, and on this account 

 I would recommend that the illuminating lens sho^dd be perfectly 

 free from chromatic and spherical aberration, and that the greatest 

 care be taken to exclude all extraneous light, both from the object 

 and from the eye of the observer. !> ^This is a judgment which every 

 advance in the construction of the optical part of the microscope, as 

 used by the most accomplished experts, has fully confirmed. 



We have no knowledge, from an inspection of the piece of 

 apparatus itself, of the construction of the compound sub-stage con- 

 denser of Bonanni (fig. 101); it does not appear to have attracted 

 much attention, and of course it was quite impossible to secure a 

 critical image by its means. It was focussed on the object merely 

 to obtain as bright an illumination as possible, in order that the 

 object might be seen at all. 



In the condenser used by Smith in his catoptric microscope 

 (fig. 113) we have the earliest (1738) known condenser, by means of 

 which a distinction between a ' critical ; image that is, an image in 

 which a sharp, clear, bright definition is given throughout, free from 

 all ' rottenness ' of outline or detail and an ' uncritical ' or imperfect 

 image could be made. It was not, apparently, at the time it was 

 first used, considered to be so important as we now know it to be ; 

 and it is probable that the mode of focussing the light upon the 

 object by its means was to direct the instrument to the sky with 

 one hand and to use the biconvex condenser with the other. In 

 1837 Sir D. Brewster writes of it with appreciation, saying that 

 ' it performs wonderfully well, though both the specula have their 

 polish considerably injured. It shows the lines on some of the test 

 objects with very considerable sharpness.' 



No advance was made on this condenser for nearly a century. 

 In 1829 Wollaston recommends the focussing of the image of the 

 diaphragm by means of a plano-convex lens of J of an inch focus 

 upon the object, and Goring in 1832 says concerning it : ' There is no 

 modification of daylight illumination superior to that invented by 

 Dr. Wollaston.' But Sir D. Brewster objected to this, contending 

 that the source of light itself should be focussed upon the object. He 

 preferred a Herschelian doublet placed in the optic axis of the micro- 

 scope. But, whilst there is a very clear difference between these 

 authorities, we can now see that both were right. 



Goring, who was also a leader in the microscopy of his day, used 

 diffused daylight, and as the lens he employed was a plano-convex 

 of | of an inch focus, the method of focussing the diaphragm was as 



