166 
Transactions of the Society. 
A crystal of picric acid may be added to all malachite-green 
solutions containing glycerin ; this removes the band in the invisible 
violet entirely (fig. 4), but it must be borne in mind that this band is 
extremely faint, even when a quartz prism is used, and I have been 
unable to detect it after the light has passed through the Microscope ; 
probably it will not pass balsam. It is rather difficult to add the 
picric acid, which is apt to throw down the dye, unless care is taken 
always to add the picric acid in crystals after the dye has all been dis- 
solved and the solution filtered. AVith a flame or lime-light spectrum 
this band is hardly to be traced, and the picric acid is quite unne- 
cessary. In any case both this and the red band may be entirely 
removed by passing the light through a blue-green glass, such as 
Messrs. Baker supply with their Microscope-lamp, and this may be 
useful for focusing with the aqueous solution where the red band is 
strong. After focusing, the blue-green glass may be removed. 
Malachite-green may also be dissolved in plain collodion, or in 
balsam, or any white varnish, and in this form it gives the blue-green 
band, brilliant and narrow, almost identical with that given by the 
glycerin solution. 
By the gradual addition of almost any acid to any of these solu- 
tions, the band may be moved down towards the red, until it finally 
coalesces with the red band and is lost. Except in this case, the 
blue-green band appears to be movable up and down the spectrum 
•pari passu with the refractive index of the solvent used, and for 
photographic purposes a solution in absolute alcohol would be the 
most useful, were it not for the small amount of visual effect and 
consequent difficulty of focusing, the band then lying almost entirely 
on the more refrangible side of F. And were it not for this same 
difficulty, a still greater shifting towards the violet might be accom- 
plished by the addition of a small quantity of cyanine. 
Almost any of these solutions may be made of sufficient strength 
for mounting between two cover-glasses in a layer little thicker than 
one of them, or in shallow cells on a glass slip. There is considerable 
advantage in mounting between glass-covers, which may then be 
inserted in the substage condenser in place of a diaphragm, or may 
occupy a position on a rotating wheel of stops. 
The advantages I venture to claim for malachite-green used in 
this way are as follows : — 
(1) It gives a field of view uniformly monochromatic. 
(2) There is more light than with chrome-copper solutions, and 
the light is more monochromatic if certain precautions are taken. 
(3) It need not be used in solution, but, if so used, a very thin 
stratum is sufficient, and no large troughs or bottles are nece^ary. 
(4) No bathing of plates in erythrosine, or cyanine, or phosphine N 
is necessary, an ordinary rapid plate being sufficiently sensitive in 
this part of the spectrum ; in fact, more refrangible light cannot con- 
veniently be used, or there would not be enough light to focus by. 
