Compound Microscope

The aberrations produced by refraction at the cover-glass and at the first surface of the front lens of the object-glass can be reduced by filling the space between the object-glass and the object, or the cover-glass, with a medium of higher refractive index than air. Amici introduced the first practical immersion lenses in 184o, using water as the immersion fluid, and improved water-immersion lenses were made by E. Hartnack in 1855. On the suggestion of J. W. Stephenson, Abbe in 1878 investi gated the matter and designed the first homogeneous immersion lenses and had these made up by the firm of Zeiss.

The apochromatic immersion object glass, in virtue of its large numerical aper ture and its almost perfect chromatic and spherical corrections, is the most nearly perfect type of object-glass constructed. Since the first apochromatic object-glasses were designed by Abbe about 1886, slight improvements in the degree of correction have been made, but no such marked im provement as that represented by the dif ference between achromatic and apochromatic object-glasses.

Object-glasses for Ultra-violet Work.--The

number of materials suitable for making lenses to be used with ultra-violet radiations is very limited. Most of the solid materials which are transparent to these radiations are unsuitable owing to their doubly-refracting properties, or else are difficult to obtain in sufficiently large pieces of good optical quality. Object-glasses for

use with ultra-violet radiations are nearly all designed as mono chromatic lenses (see of Object-glasses).

An achromatic object-glass made of quartz and fluorite was produced in 190o by von Rohr for ultra-violet work; this lens had a focal length of about 4 mm. and a numerical aperture of 0.3. In 1904 the firm of Zeiss introduced a series of lenses, made from fused quartz, for use with monochromatic ultra-violet light, having numerical apertures up to about 1.25. In 1926, an object glass of novel design for use as a water-immersion lens, and an other for use as a homogeneous immersion lens, were designed and computed by the British Scientific Instrument Research Asso ciation. Lenses made up to these formulae by the firm of R. and J. Beck have numerical apertures of 1.14 and 1.2 respectively, and are fully corrected for use with the monochromatic ultra violet radiation for which they were computed. The resolving powers of these objectives are equivalent to those obtained with lenses of aperture 2.28 and 2.4 when they are used with green light.