224 E. M. NELSON ON HISTORIC MICROSCOPE. 



could be easily accounted for by the fact that although the single 

 lenses had less power, they gave much sharper images, because the 

 aberrations of the uncorrected lenses were not magnified. 



Knowing this, he set to work to lift the compound microscope 

 out of the mire ; and so thoroughly did he do it, that 60 years 

 afterwards I find it referred to in the following terms : — " In the 

 opinion of the ablest judges, it is incomparably superior to them 

 all." 



In 1837, that is, 100 years after its invention, Sir D. Brewster 

 says : — " 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." What a 

 report for a philosophical instrument 100 years old ! Can anyone 

 bring me a microscope twenty years old that will show the lines on 

 some of the test objects of to-day " with very considerable sharp- 

 ness ? " What philosophical instrument can you point to that has 

 stood the scorching heat of 100 years of improvements without 

 being demolished ? Dr. Smith, seeing the impossibility of doing 

 anything with uncorrected lenses, devised a reflecting microscope, 

 the arrangement of the mirrors being somewhat similar to that in 

 a Cassegrain telescope. The rays of light, condensed on the object 

 by a substage condenser, passed on to a concave speculum, 

 which took the place of the object glass of the compound micro- 

 scope ; they were reflected back to a convex speculum, which 

 brought them to a focus where they were examined by a biconvex 

 eye-glass. There was a stop placed in the tube near the hole 

 in the convex speculum to prevent any direct rays entering the eye. 

 In speaking of the condenser, he says, " that this lens should be 

 just so broad as to subtend the opposite angle to that which the 

 concave speculum subtends at the object." This is, perhaps, one 

 of the most extraordinary statements in the annals of historic 

 microscopy. This principle, viz., that of placing the object in 

 the conjugate foci of the objective and condenser, and of making 

 the angle of the illuminating cone equal to the angular aperture 

 of the objective, 1 have enunciated here for some years past, and 

 have repeatedly exhibited objects under that illumination both 

 here and elsewhere, little thinking that Dr. Smith had laid down 

 the same principles 150 years ago. It would, indeed, be a grand 

 thing if glasses nowadays were well enough corrected to permit of 

 their being illuminated by solid cones of light equal in angle to 



