434 Transactions of the Society: 



Microscopes ; he gives all the data of No. III., and says that " It 

 will be observed that there is no great obliquity in either reflexion ; 

 and I should imagine that the image formed in such an instrument, 

 it' well executed, would be very distinct." 



We now pass on to the simple dioptric Microscope, which 

 consisted of a single equiconvex lens much stopped down. Some of 

 these had very short foci ; one made circa 1760 measured ^5 in-, 

 and the aperture was a mere pin's point. Some have expressed 

 astonishment at those astronomers who used the aerial telescope 

 of some 200 feet focus, mounted without a tube at the top of a 

 pole ; but allow me as a microscopist to express equal astonish- 

 ment at those early naturalists who worked with very high power 

 simple Microscopes. 



Microscopes in those days were often sold as "single" and 

 " double," which means that in the same box with the compound 

 Microscope a set of simple lenses was supplied as well. The Ben- 

 jamin Martin Microscope referred to above had six of these simple 

 Microscopes. As to the low powers, there is no difficulty in under- 

 standing how to use them ; the compound body is unscrewed from 

 the limb, and the simple Microscope takes its place ; but with the 

 high powers a difficulty arises because the focal point of the minute 

 lens is inside a sort of cup (not a lieberkuhn) on one side, and in 

 a tube on the other ; it is, therefore, quite impossible to focus it 

 either on a modern glass mount or upon one of the ancient ivory 

 sliders ; an object held in the stage tweezers might be focused 

 inside the cup, but then how is it to be illuminated ? In a Cuffs 

 Microscope the simple lenses are mounted in lieberkiihns, but the 

 lenses are placed at the end of a small dome, which brings the lens 

 to the level of the rim of the lieberkuhn, therefore with one of 

 these an ordinary slide may be examined. 



Strictly speaking, we have brought the investigation of the three 

 principal forms of Microscopes up to the date which was mentioned 

 at the beginning, viz., that of the achromatisation of the Microscope ; 

 we ought, however, to be allowed some little latitude for overlap, 

 because achromatism took a little while to become established, 

 during which time not only did the old forms remain, but great 

 improvements were effected in the non-achromatic Microscope. 

 Tbe greatest of all these improvements was the introduction of 

 Wollaston's doublet in 1829. The ratio of the foci of these lenses 

 was 3 : 1, placed at their best distance apart as discovered by trial ; 

 in brief, it was a Huyghenian eye-piece turned up-side down.* 

 Later it was found that a distance of the lenses apart equal to the 

 difference of their foci gave the best results, but it seemed that a 

 precise ratio of 3 : 1 for their foci did not matter much. Coddington 



* Wollaston was dying when he published the account of this Microscope, and 

 said that his publication was premature, as his experiments with it were not 

 completed. 



