NEW MICROSCOPES—SEIDEL AND WINTER 205 
ment of geology, Harvard University, have very convincingly demon- 
strated a reduction in these theoretical limits of resolution and visibility 
with their instruments, designed for use in the visible light region of 
the spectrum. 
The Graton-Dane microscope is mounted on a 360-kg. steel founda- 
tion bed which, in turn, is supported by six rubber-in-sheer marine- 
engine mountings—this for the purpose of eliminating all vibration 
and insuring stability of parts, two factors upon which both men have 
laid great stress. Any type source, such as the carbon arc, metallic arc, 
incandescent filament, Point-O-Lite, mercury vapor, or any of the 
special forms of monochromators, can be used for illuminating the 
specimen with direct and dark-field transmitted, vertical and oblique 
reflected, or polarized light. The image beam itself follows a straight- 
line path in passing from the objective, the objective ranging anywhere 
from the shortest to the greatest in working distance, through the tube 
to the ocular, as few lenses as possible being placed in its way. The 
spiral-cut rack and pinion which moves the stage and substage assem- 
bly in longitudinal tracks or guides can be operated by hand or by an 
electric motor and is independent of the fine adjustment, also motor- 
driven, which moves only the objective and the carriage carrying the 
objective. Whereas manual operation of the fine adjustment which is 
100 times more sensitive than that of the ordinary instruments neces- 
sitates 500 turns of the knob to move the objective a distance of but 
1 millimeter (an adjustment calculated to require a time period of 
25 minutes), by means of the motor it is possible to move the objective 
at the rate of 0.01 mm. per second or 0.004 mm. per second, depending 
upon which of the two speeds is desired, rapid motion being used when 
the image appears considerably out of focus and decreased speed being 
used when the image seems to be reaching a point of perfection 
Resolution up to 6,000 diameters and magnification up to 50,000 
diameters have been achieved with this high-precision microscope 
which photographs or enables observation of both opaque and trans- 
parent preparations; in fact, polishing scratches measuring in width 
but one-tenth the wave length of light used have been clearly distin- 
?The mechanism governing the fine adjustment was completely redesigned after it was 
discovered that changes in the lubricant, used for gear threads and carriage bearings, 
seriously affected the precision of the instrument. Using a principle suggested to him by 
R. W. Vose, formerly of the Harvard Engineering School, Dr. Dane built and assembled 
a new fine-adjustment drive so designed that, as Dr. Graton describes it, “all that part 
of the mechanism which actuates the slowest, and therefore the most sensitive, part 
of the motion operates not through gears or screws, but through the differential flexing of a 
train of spring-bronze strips, which have the double advantage of avoiding all chance for 
play or backlash and of needing no lubrication whatever. Interferometer tests with the 
new element in place give practically ideal readings as compared with the theoretical: 
the deviations are very much smaller than those recorded in our original paper, page 372. 
The operation of the fine-focusing mechanism selectively by hand knob or by motor-drive, 
and the slowness of motion, and hence the precise control over focus are the same in the 
new design as in the old.” 
