NEW MICROSCOPES—SEIDEL AND WINTER 209 
then carried up the tube in parallel rays through 21 light bends to the 
ocular, a tolerance of less than one wave length of visible light only 
being permitted in the core beam, or chief ray, of illumination. Now, 
instead of the light rays starting up the tube in a parallel fashion, 
tending to converge as they rise higher and finally crossing each other, 
arriving at the ocular separated by considerable distance as would be 
the case with an ordinary microscope, in the universal tube the rays 
also start their rise parallel to each other but, just as they are about te 
cross, a specially designed quartz prism is inserted which serves to 
pull them out parallel again, another prism being inserted each time 
the rays are about ready to cross. These prisms, inserted in the tube, 
which are adjusted and held in alignment by micrometer screws of 100 
threads to the inch in special tracks made of magnelium (magnelium 
having the closest coefficient of expansion of any metal to quartz), are 
separated by a distance of only 30 millimeters. Thus, the greatest 
distance that the image in the universal is projected through any one 
media, either quartz or air, is 30 millimeters instead of the 160, 180, 
or 190 millimeters as in the empty or air-filled tube of an ordinary 
microscope, the total distance which the light rays travel zigzag fashion 
through the universal tube being 449 millimeters, although the physical 
length of the tube itself is 229 millimeters. It will be recalled that if 
one pierces a black strip ef paper or cardboard with the point of a 
needle and then brings the card up close to the eye so that the hole is 
in the optic axis, a small brilliantly lighted object will appear larger 
and clearer, revealing more fine detail, than if it were viewed from the 
same distance without the assistance of the card. This is explained 
by the fact that the beam of light passing through the card is very 
narrow, the rays entering the eye, therefore, being practically parallel, 
whereas without the card the beam of light is much wider and the 
diffusion circles much larger. It is this principle of parallel rays in 
the universal microscope and the resultant shortening of projection 
distance between any two blocks or prisms plus the fact that objectives 
can thus be substituted for oculars, these “oculars” being three matched 
pairs of 10-millimeter, 7-millimeter, and 4-millimeter objectives in 
short mounts, which make possible not only the unusually high mag- 
nification and resolution but which serve to eliminate all distortion as 
well as all chromatic and spherical aberration. 
Quartz slides with especially thin quartz cover glasses are used when 
a tissue section or culture slant is examined, the tissue section itself also 
being very thin. An additional observational tube and ocular which 
yield a magnification of 1,800 diameters are provided so that that por- 
tion of the specimen which it is desired should be examined may be 
located and so that the observer can adjust himself more readily when 
viewing a section at a high magnification. 
