94 THE MICROSCOPICAL NEWS. 
spheres” having been sold at the shops of the Athenian grocers ; 
Seneca, the Roman philosopher, who died A.D. 65, described the 
magnifying capacity of “globules of glass filled with water ;” and 
Ptolemy, the Alexandrian astronomer, who lived about the same 
time, wrote a treatise on Optics, in which he described the principle 
of refraction in glass. In the British Museum there is a lens 
made of rock crystal, which was found amongst the ruins of 
Nineveh. The compound microscope appears to have been 
first invented by Zacharias Jansen about the year 1590, and the 
early transactions of the Royal Society, which was established in 
1660, contain many records of improvements in the instrument, 
and discoveries by its means. The obstacles that retarded the 
introduction of the achromatic principle were very great. Sir 
Isaac Newton had announced in his “ Optics” the proposition 
that, as all refracting substances caused prismatic colours to 
appear, ‘‘no improvement could be expected in the refracting 
telescope,” and the ‘Newtonian reflector” became the most 
approved instrument of the astronomer; Newton also invented a 
reflecting microscope ; and after the Dollonds had produced their 
excellent 2? inch achromatic object-glasses, the applicability of 
the principle to the microscope was disputed, owing to the small 
size of the microscope objective, and the difficulty of grinding 
small surfaces to the necessary curves, and also to the idea then 
prevalent that it must be an imitation of the telescope objective, 
and not something sw generis. ‘The discovery of the achromatic 
principle in its applicability to object-glasses for telescopes, was 
first made by John Dollond in 1757—for which the Royal Society 
awarded him their Copley medal; but it was not until the year 
1823 that any successful attempt was made to apply this principle 
to the microscope, when Selligues and Chevalier, of Paris, pro- 
duced achromatic combinations of two or more pairs of lenses, 
each pair being composed of a double-convex of crown glass and 
a plano-concave of flint, the latter correcting the aberrations of the 
former, whilst a capacity for refraction remained. ‘The first 
English achromatic objective was made by William Tulley in 
1824. Amici, in 1827, produced an improved combination of 
three pairs, and soon afterwards J. Jackson Lister, instructed by 
the researches of Sir John Herschell and others, produced a new 
combination of lenses, in which he used Canada balsam to cement 
the lenses of each pair, and prevent the great loss of light by 
reflection from their surfaces. In 1837 Mr. Ross introduced the 
“correction collar” to the higher powers, an improvement rendered 
necessary on account of the excellence of the i images produced by the 
new achromatic lenses, and the interference of the “cover-glasses” 
with the definition, In 1850 Mr. Lister introduced the improvement 
of the ¢viple back and single front lens, (separated by a double middle ), 
