NOTES AND MEMOEANDA. 
275 
Academy, will be found at p. 217, has contributed some further remarks 
on the subject to the ‘ Bulletin of the Mineralogical Society of France,’ 
of which the following is a translation, omitting those parts which 
repeat what has already appeared in the ‘ Comptes Eendus.’ 
“ The eye-piece which enables the observer to make sure that one 
of the faces of the crystal has its projection perpendicular to the zero 
plane (described in the previous article), has been slightly modified 
with the view of obtaining greater sensibility. It is composed of a 
cylinder of flint of 6 centimetres long, to the middle of which is fixed, 
by Canada balsam, a plate of crown of the ^th of a millimetre thick. 
The flint having a greater, and the crown a smaller refractive index 
than that of the balsam, it will be seen that the upper part of the 
cylinder being placed at the focus of the upper lens of the eye-piece, 
two reticles will appear very close and parallel, and the interior of 
these two reticles will be illuminated if the face of the crystal has its 
projection perpendicular to the zero plane of the microscope. How- 
ever little the crystal is turned to the right or left from this position, 
the part comprised between the two reticles will cease to be illumi- 
nated, whilst the exterior part will be more strongly illuminated 
either on the right or left according as the crystal has been turned. 
The possible error is given by the value of the angle whose sine 
is ; this angle is less than 10', and as two readings can be made by 
turning the crystal to the right or left successively until the interval 
between the two reticles is completely darkened, the error is reduced 
to 5'. 
Measurements made on crystals of to of a millimetre have 
given results correct within 6'. 
The smaller the face of the crystal is, the greater, of course, will 
be the sensibility of the process. If a face is observed which is not very 
small, this face will reflect light into the microscope obliquely to the 
optic axis of the apparatus even when the projection of the face of the 
crystal on the horizontal plane is perpendicular to the zero plane, 
and this oblique light will destroy the clearness of the phenomenon ; 
whilst if the face is very small, all the light reflected by it will be 
sensibly parallel to the zero plane of the microscope. Moreover, 
when the face of the crystal is not very large, its image is seen 
in the microscope on both sides of the double reticle when this 
face has its projection perpendicular to the zero plane ; but on “a 
slight rotation of the stage to one side or the other of the correct 
position, the image disappears either to the right or the left of the 
double reticle, and remains visible on one side only. This dis- 
appearance of one half of the image in conjunction with the extinction 
of the part comprised between the two reticles renders the observation 
very easy. It is sufficient therefore to employ magnifying powers 
proportionate to the size of the crystal, so that the sides of the face 
observed should appear in the microscope to be about 2 mm. The 
difficulty in the use of high powers is their short focus, which pre- 
vents good illumination. Crystals of about mm. may be 
measured, but for smaller ones the process ceases to be applicable.” 
M. Bertrand also explains his mode of illumination to be to place 
a luminous slit of about 30 centimetres long before the microscope 
