A Mode of altering the Focus of a Microscope. By M. Govi. 207 
to obtain with them what we could hardly hope to get with solids. 
Hence it is only necessary to employ a liquid layer placed, in a 
sort of cup with a transparent bottom, beneath the objective. or 
here we have a refractive plate whose thickness we can increase or 
diminish, and whose surfaces keep up constantly the same relation 
with each other and with the optical axis of the microscope. 
If then one establishes between the objective and the object 
a reservoir sufficiently large (in order to avoid capillary attrac- 
tion), closed below by a plate of glass with horizontal surfaces 
sufficiently parallel, and if then one introduces a liquid of the index 
m, which one may vary the level of at will by aid ofa plongewr, or 
by means of a communicating vessel filled with the same liquid, 
one may always by variations in the weight of the refractive layer 
bring to the same vertical distance objects whose real distances 
differ by quantities more or less considerable. The limit of these 
accommodations is given by the value of d, calculated from the 
formula already given. And as one hardly has recourse to liquids 
whose index of refraction is less than 1+3335 or more than 2°000 
this limit would always be comprised between about a quarter and 
a half of the maaimum thickness of the liquid contained in the cup. 
It must be understood that it is not requisite to take account, 
in this valuation of d, of the constant displacement produced by the 
layer of glass and by a layer of liquid which it is well to have 
above it in order that the variations of thickness may take place in 
the same refractive substance. 
In cases where the objects to be observed are directly plunged 
in a liquid, this liquid might be made to serve the focalization of 
their images. 
The extreme mobility of most liquids demands great steadiness 
in the “cup” which carries them; without this the agitation of 
their surfaces would prevent the proper formation of images, just as 
it not unfrequently happens in the mercury baths that are used for 
astronomical purposes. If there are no trepidations, the observation 
of transparent or opaque objects is conducted as easily through the 
liquid layers as through the air, and the loss of light which results 
from successive reflexions does not materially affect the clearness of 
the images. 
A slight defect of parallelism between the inferior surface of 
the “cup” and the upper surface of the liquid should not affect the 
exactitude of the focus, whilst perfect horizontality of this latter 
gives this slight defect a constant value, whatever be the thickness 
of the liquid layer interposed. It produces only in this case a 
slight lateral displacement of the images, which, being the same for 
all, alters in nothing the value of their relative distances.—A Paper 
read before the French Academy, February 19, 1877. 
