WELCKER, ON MICROSCOPIC MEASUREMENTS. 241 
spheroidal bodies or convex solid corpuscles—as, for instance, 
a particle of glass—may exhibit the brilliancy on the depres- 
sion of the tube when such bodies are contained in a medium 
possessing stronger refractive power than themselves. Thus 
particles of glass contained in oil of aniseed, appear brilliant 
on the depression of the tube, exhibiting, in fact, precisely 
the same optical conditions as a cavity in glass or an air- 
bubble in water. 
The following observations are intended to point out the 
further applicability of a methodical elevation or depression of 
the object-glass. 
1. Measurement of the vertical diameter of microscopic 
objects. 
The idea readily suggests itself, that the height of micro- 
scopic objects may be estimated under the microscope by the 
determination of the distance passed through by the object- 
glass, when focussed on the apex and base of the object. 
When the value of a turn of the screw of the fine-adjustment 
is known, and can be estimated by proper graduation of the 
head of the screw, it is very easy to determine how much 
the tube has been raised or depressed in this operation. 
This mode of determining the altitudinal diameter is given 
by Harting,* and several authors have stated the values of 
vertical diameters obtained in this way. But the vertical 
diameter of microscopic objects cannot be thus immediately 
determined by the amount of movement of the tube, which 
movement gives us the vertical diameter of the object, as 
affected by the difference between the refractive power of the 
air and that of the object under view. 
The correctness of this statement is obvious, from the 
simple consideration, as first pointed out by V. Mohl,+ that 
the rays of hght proceeding from an object covered with 
glass enter the objective in a different direction, and 
consequently require a different (more elevated) position of 
the tube, than do those of an uncovered object. When 
layers of equal thickness of various highly refracting 
substances are examined, it will be found that the distance 
traversed by the tube when adjusted to the upper and lower 
surfaces respectively, is less in proportion to the greater 
refractibility of the substance examined. The length of 
‘movement. of the tube, therefore, employed immediately as a 
measure of the vertical thickness, would lead to an under- 
* ©Das Mikroskop.’—Germ. Transl. 
+ ‘Mikrographie,’ p. 159. 
