THE POLAEISCOPE. 
161 
until it appears brightest, and then turn round the prism to 
examine the different colours given by selenite. The brightest 
colours to be obtained are either a rose-pink and green, or sky- 
blue and gold-yellow. In a specimen I measured, I found 
the brightest blues and yellows to be obtained with a plate 
of thickness about 0*0030 inch, and the best green and pink 
with a thickness 0*0068 inch. By taking three plates, two of 
these thicknesses and one of thickness between the two, and 
combining them in any way by using two or more at once, a 
great variety of shades may be produced. In using a selenite 
plate for examining objects under the microscope, it must be 
borne in mind that the brightest colours are to be obtained only 
in two positions of the selenite relatively to the fixed polarizer. 
Therefore if the polarizer and analyzer do not both revolve, care 
must be taken to mount the selenite, so that on being laid on 
the slide, it must be in the position relatively to the fixed 
polarizer to give the best light. This is easily done by trial. 
First adjust the movable polarizer until the field is black, then 
introduce the selenite, and turn it round until it is at its brightest. 
Fix it in this position. A plate of selenite is also useful in ex- 
amining doubly refractive crystals, when polarizer and analyzer 
do not both revolve. The most beautiful results are to be 
obtained with the greatest ease with the polarizing apparatus 
and microscope. 
Drop on a slide a drop of a solution of any salt crystallising 
in any form not of the cubical system. Then either heat the slide 
gently over a flame, or allow the salt to crystallise at ordinary 
temperatures. Grroups of crystals of all varieties of patterns will 
be formed on the slide, and when viewed by polarized light will, 
if they are of the proper thickness, be invested with the most 
brilliant hues. With any particular salt, a few trials will soon 
show the best plan of proceeding. First evaporate the drop 
rapidly. Examine it, and see if it is the thicker or the thinner 
crystals, the larger or the smaller ones that give the best colours. 
If the small thin crystals give the colours best, put a drop of 
water on the slide rather larger than the drop of solution before 
used ; hold the slide a little sloping, and heat it over a flame. 
If larger crystals are best, make a saturated solution, and put 
a thick drop on the slide, and allow it to evaporate slowly. 
Another plan is to place on the drop of solution a warm thin glass 
plate. The crystals are then sometimes flat, and the slide may 
be preserved by running a little gold size round. A slide can 
also be preserved by castor oil. A little is placed over the 
crystals and gently heated. A thin glass circle is then placed 
on and pressed down. This is, in the usual way, secured with 
marine glue. A long list of substances viewed with advantage 
by polarized light is given at the end of Dr. Carpenter’s ex- 
