and on their application to Telescopes and Microscopes. 105 
an enlarged form ; and I constructed one about J^een feet in 
length, with an achromatic object-glass, which produced very 
superior effects Since that time, I constructed another, with 
a metallic reflector, which was 48 feet in length* 
The advantages of large microscopes over small ones, may be 
considered in reference, 
1. To the imperfections of the glass employed. 
To the spherical form into which it is ground. 
3. To the adjustment of the axes of the lenses. 
4. To the method of illuminating the object. 
5. To the examination of objects placed in cavities ; and, 
6. To the examination of objects whose parts are placed at 
different distances from the instrument. 
In making this comparison, we shall suppose that the object- 
glass of both microscopes intercepts the same portion of the 
sphere of light, which diverges from the object under examina- 
tion. 
Ist^ As the veins and irregularities of glass have a definite 
magnitude, a lens of a small aperture will be much more liable 
to have its image injured by any accidental flaw, than one of a 
large size, and the same may be said of the small pits and 
scratches which often remain even after the most careful polishing. 
i^d, In the operation of grinding the object-glasses of stnall 
microscopes, the optician works at random, and has the power 
neither of giving them a correct spherical figure, nor of adjust- 
ing the axes of their opposite surfaces ; whereas in larger lenses, 
these operations are completely under his controul. 
2d, One of the principal points to be attended to in the con- 
struction of compound microscopes, is the coincidence of the 
axes of the lenses of which it is composed. This adjustment is 
seldom made, and indeed is not very practicable when the lenses 
are small. In the enlarged form, however, the axes of the lenses 
* M. .®pinus, in the Nova Acta Petropol. tom. ii. p. 45. proposes that the dis- 
tance of the object from the object glass should be three, four, or five inches, or 
even half a foot or a foot, in order to allow the light to fall upon the object ; arid 
he describes a microscope which he had constructed on this principle, With an achro- 
matic object-glass a little less than three feet in focal length. The aperture of the 
object-glass was about an inch, and the distance of the object from the object-glass 
seven inches. 
