THE OPTICAL ELEMENTS OF THE MICROSCOPE LENS SYSTEM. 51 
The following summary of the corrections required in the different forms 
of objectives is given bySpitta* in his recent book on microscopy, in which 
the subject of the optical system of the microscope is treated in an unusually 
clear and non-technical manner : 
" (a) An ordinary low-power achromatic must be corrected for 
(1) Primary spherical aberration. 
(2) Elimination of coma. 
(3) Primary color. 
(b) An ordinary high-power achromatic, in addition, must be approxi- 
mately corrected for 
(4) Secondary spherical aberration (spherical zones) for the pre- 
ferred color. 
(c) Semi-apochromatics, besides fulfilling the above four conditions in 
a highly perfect manner, must be made free from 
(5) Primary spherical aberration for a second color (and approxi- 
mately for all colors) ; and 
(6) Should be computed so as to give equal magnifications for all 
colors when used with compensating eye-pieces ; while 
(rf) The full apochromatic must further show 
(7) Freedom from the secondary spectrum." 
On high-power dry objectives correction collars are used to correct for 
variability in thickness of the cover-glass. The use of such a correction 
collar requires some practice, as the correct thickness is judged directly by 
the appearance of the image. To facilitate such tests Abbe devised a test 
plate which consists of a thin silver grating mounted between an object- 
glass and a wedge-shaped cover-slip, the thickness of which for different 
points is indicated by a scale reading to hundredths of a millimeter. In 
making the test the silver strips are brought to focus and the images in the 
central part of the field of view examined as to their quality and the effect 
of central and oblique illumination on their definition. If the objective be 
properly adjusted to cover-glass thickness and be correctly constructed, the 
edges of the silver strips should appear perfectly sharp without hazy outline, 
both with central and with oblique illumination. If this be not the case 
the spherical correction is more or less imperfect. The degree of chromatic 
correction is estimated by the character of the color fringes produced by 
oblique illumination. Good achromatic objectives should display in the 
center of the field only narrow bands in the complementary colors of the 
secondary spectrum, yellow-green on one side and violet to rose color on the 
other. With apochromatic objectives these colors should not be present. 
After some practice the eye recognizes at once the defects produced in the 
image by improper thickness of cover-glass ; these are then to be eliminated 
either by use of the correction collar of the objective or by lengthening or 
shortening the draw- tube of the microscope. 
The numerical aperture can be determined by a number of different 
methods. A simple and accurate method is by means of the Abbe aper- 
tometer, which is a thick polished glass semicircle terminated on its straight- 
edge side by a surface at 45 to the normal, which reflects light rays, entering 
along the circular rim of the disk up into the microscope. By means of 
*E J Spitta, Microscopy, London, 1909. 
