THe Microscope. 203 
genius and untiring ambition. To great theoretical and prac- 
tical knowledge of the science of optics, he united mechanical 
and inventive genius and marvelous skill of eye and hand. 
While still in the service of Mr. Spencer he devised the form of 
cover correction for objectives, in which back and middle com- 
binations have a rectilinear motion only, and the front remains 
entirely stationary. In 1853 he invented and patented his solid 
eye piece. In 1858 he made his first immersion objectives, 
though of course, this plan was not original with him. In 4858 
he constructed objectives with two fronts, one to be as an im- 
mersion and the otherdry. In August, 1873, he made the great 
step forward which placed him at the head of his profession, the 
Columbus of a new era of microscopy. He made an immersion 
one-tenth with an aperture greater than that corresponding to 
infinitely near 180 degrees in air. It was a three-system lens 
and had an aperture of more than 110 degrees in balsam or 1.25 
N. A. The same month he made his first lens of the duplex 
front formula one-fifth glycerine immersion of 110 degrees bal- 
sam angle. Both passed into the possession of the Army Med- 
ical Museum at Washington, and both were practically homo- 
genous immersion lenses, as they gave the very best results 
when immersed in soft balsam, which had been brought as 
nearly as possible to the same index of refraction as the crown 
glass of which their front lenses were made viz. 1.525. The 
importance of this bold step and its influence upon the progress 
of microscopy can scarcely be estimated at this time, but it is 
certain that it was the cause of a revolution of opinion and prac- 
tice among users and makers of microscopes all over the world. 
Mr. Tolles was the inventor of many other devices and ap- 
pliances for the microscope. In 1866 he invented and patented 
his stereosopic binocular eye piece; in 1878 he received two 
patents for improvements in the microscope stand. For years 
his stands were models of elegance, convenience and stability, 
and his mechanical stages are, I believe, unequalled to-day for 
delicacy of construction and perfection of working. In addition 
to his work on the microscope he made some telescopes remark- 
able for their short focal length in proportion to the diameter of 
object glass and for their remarkable defining and penetrating 
power, a small instrument with an objective only one inch in 
