226 
CORRESPONDENCE. 
by means of reflecting mirrors. It had now become apparent that 
an object table was necessary to complete the microscope, which 
likewise demands an increase in the solidity of the stative. In 
this manner, as well as through the improved lenses the magnify¬ 
ing power had been increased nearly 700 times, so that better and 
more perfect focusing arrangements became absolutely necessary. 
The first simple microscope with an object table was made by 
Cuff, in 1755. This microscope had better arrangements for more 
accurate focusing. In the last half of the last century, as well as 
in the first half of this century, microscopes were made more or 
less similar to those of Cuff. Joblot, in France, in 1718, improved 
the optical apparatus by uniting two biconvex lens into a double, 
which Euler first made practical and brought into a condition suit¬ 
able for general use. 
This was the point of development to which the simple mico- 
scope had arrived at the end of the first period in the history of 
microscopy. It deserved unreservedly the preference over the 
compound microscope. For this reason all the most important 
micrographic researches were carried on by the aid of the simple 
microscope. Many doubted the possibility of perfecting the com¬ 
pound microscope so that it could replace the simple one. 
Let us examine the development of the compound dioptric mi¬ 
croscope. 
In what form, the compound dioptric microscope came from 
the hands of the discoverer, it is impossible to declare with cer¬ 
tainty, yet, we can safely affirm, that it consisted of two convex 
glasses. This arrangement was used until the middle of sixteenth 
century, then a third lens—convex, was added, or two plain con¬ 
vex lenses. Its power was very small and as a maximum about 
80 lines. One hundred and forty diameters could be attained by 
lengthening the tube in an extraordinary manner. 
Among the oldest and most famous compound microscopes 
were those made by Robert Hooke, the Roman Eustachio Divini. 
On the principle of JDivini, Johann Franz Grindl of Ach, made, 
in 1685, a. microscope which had six plain convex lenses arranged 
in pairs, with the convex surface turned toward each other. Over 
the ocular was a perfectly plain glass. The compound microscope 
