SCIENCE AND INDUSTRY. 
Further improvements were brought about by a combination of forces 
in Germany. New substances were used in glass-making at Schott’s 
elassworks; and, from calculations by E. Abbe, the firm of Zeiss pro- 
duced the apochromatic objective in 1886, the most perfect type of 
objective yet made from the new Jena optical glass, in conjunction with 
minerals of special optical qualities like fluorite. 
In these the correction is so made that three colours are combined 
in the one focus, and spherical aberration is removed for two colours. 
Some of these apochromatics have as many as twelve lenses, and are 
consequently very expensive. The orange and green may be combined, 
and the combined result made to agree with the violet. 
Even in these objectives there is a slight difference in the size of the 
red and blue images—the red being smaller. This is controlled by a 
special eye-piece to go with them, in which the red image is magnified 
more than the blue, and so compensation effected. 
The objective is the most essential part of the microscope, and is 
used to produce a real magnified image. The difference from the simple 
microscope is due to the difference in situation of the object looked at 
and the focus. In the simple lens, the object is nearer the lens than the 
focus. If it is put further away than the focal length, we shall also 
get an inverted image. In the objective, the focus of the whole 
is greater than that of the front system, hence the object is at a distance 
from the front system greater than its focal length (7.e., at the focus of 
the whole system), and hence a real inverted image. By the use of an 
eye-piece of simple construction the diverging rays from the objective 
forming the image must be collected together so that they may enter 
the eye. Hence the commonest type of eye-piece, known as the Huy- 
genian consists of two plano-convex lenses mounted together, their flat 
faces uppermost, the lower one acting like a simple reading glass, and 
collecting the diverging rays from the object image by being placed a 
little nearer the objective than the plane in which the image is formed, 
‘and the eye lens being used to observe and magnify the image. This 
permits results which are impossible with the simple microscope. The 
real image can be photographed, can be measured, and can be physically 
altered by polarization, by spectrum analysis of the light employed by 
absorbing layers, &e. 
The two systems of lenses—objective and eye-piece—are mounted at 
the ends of telescopic tubes (= the body), which should be capable of 
alteration by rack and pinion work. The body is fastened by a limb 
to the head of a pillar, resting on a solid foot, either a horseshoe mass 
or a tripod stand. At the junction of the pillar and limb, at right angles, 
is fastened the stage, usually fitted below with what are sub-stage fittings 
to hold a third optical part known as the condenser; and below that a 
mirror, plain one side and concave the other, used for reflecting the 
light to the condenser, which focusses it on the object to be examined, 
if that is to be done by the use of transmitted light. 
Taking the normal visual distance as 10 inches, a lens that has a 
focal length of 1 inch will increase the angle of vision 10 times, and we 
say that the lens magnifies 10 times or 10 diameters (written for short, 
x 10). Similarly a lens with a focal length of 4” = 60, and a 
302 
