206 A Mode of altering the Focus of a Microscope. By M. Govi. 
mediate lens (as in the parfocal meroscope of Porro). In all these 
cases, no matter how much care may have been taken in the con- 
struction of the apparatus necessary for these alterations, it is 
almost impossible to avoid very slight deviations of the optical axis 
of the microscope, and hence we cannot count on the perfect exacti- 
tude of comparisons which demand an absolute invariability of the 
direction of this same axis. If instead of changing the focus by 
means of altering the optical apparatus we obtain the alteration 
by elevating or depressing the object itself, it may happen, and it 
does occur often enough, that the masses to be displaced being con- 
siderable, the displacements occur irregularly by a series of jerks, 
with flexion of the object, and hence with an uncertainty or an 
alteration of the length to be measured. Moreover, one can hardly 
focus one of the extremities of a scale without at the same time 
altering the focus of the other end, which causes a loss of time, and 
prolongs beyond measure operations which ought to be rapidly 
performed. 
It was therefore desired to find a means of altermg promptly 
the vertical focus of the microscope within certain limits, without 
having to fear either a change of direction of the optical axis of the 
instrument or any alteration whatever in the length to be measured. 
It was then necessary to consider how this could be done without 
interfering either with the optical part of the instrument or with 
the object. And it seemed almost impossible. 
But on reflecting, it appeared that the interposition between 
objective and object of a medium more refractive than air, bounded 
by plane and parallel faces normal with the axis of the microscope, 
would affect the object. Thus there is an apparent raising of 
the object : 
where d is the amount of elevation produced, e the thickness of the 
medium introduced, and n its index of refraction compared either 
with the air or a vacuum. 
It will suffice in fact to place beneath the objective a plate with 
plane and parallel surfaces, and of variable thickness, to bring 
readily into the focal plane of the eye-piece the image of objects 
situated at different distances in front of the object-glass without 
altering the position of either the objective or the object. But it 
was, if not impossible, at least extremely difficult to obtain solid 
plates with plane and parallel faces and with a thickness which 
could be altered at will, which withal were perfectly homogeneous 
throughout, and maintained their perpendicularity in their initial 
inclination in relation to the optical axis of the microscope. 
Fortunately a well-known property of liquids—that of leaving 
their surface during equilibrium perfectly horizontal—permits us 
