ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
127 
(4) Photomicrography. 
Monochromatic Light for Photomicrography.* — Mr. A. D. Pretzl 
enumerates the chief light-filters thus : — Zettnow’s chromate of copper, 
ammonio-sulphate of copper, Fehling’s solution, and potassium bichro- 
mate, besides signal green, chromium green, and other coloured glasses ; 
although the light they allow to pass is by no means monochromatic, 
but practically the whole of the spectrum with certain portions dampened 
down. In order to make the action of screens thoroughly understood, 
the following tables of reference are necessary : — 
Fraunhofer 
Wave- 
Fraunhofer 
Wave- 
Lines. 
Length. 
Lines. 
Length. 
A 
A 
A . . 
... 759 
E . . . . 
. 527 
B . . 
... 687 
F . . . . 
. 486 
C . . 
... 656 
G . . . . 
. 430 
D . . 
... 589 
H . . . . 
. 397 
Distribution of Colours (after Listing). 
a 
A 
Bed . . 
. . 723-647 
Bright blue 
491-455 
Orange 
. . 647-586 
Blue violet 
455-424 
Yellow . 
. . 586-534 
Violet . 
424-397 
Green . 
. . . 534-491 
Zettnow’s 
cupro-chromate filter allows light from 
wave-length 
570-550 to pass, and is made by dissolving 160 grains of pure dry 
cupric nitrate and 14 grains of chromic acid in 250 c.cm. of water. The 
thickness of liquid should be 1 cm. The ammonio-sulphate of copper 
allows light from 475-400 wave-lengths to pass, and this is prepared by 
dissolving finely powdered sulphate of copper in four times its weight of 
liquid ammonia (sp. gr. 0*96). When diluted or with a thin stratum of 
solution, light of 515 A will get through. 
Fehling’s solution cuts off the extreme red and the ultra-violet. 
The potassium bichromate absorbs the violet, blue, and bluish-green, 
according to its strength and thickness. 
Landolt’s filters, as used for polariscopic work, should be equally 
good for microscopic work, and the following directions for making them 
may be useful. 
Bed filter . — Crystal violet, 5 BO, 0*05 grm. should be dissolved in 
a little alcohol and diluted to 1000 c.cm. with water, in a trough 20 mm. 
thick ; this gives a red band with a broad blue violet baud, which can 
be absorbed by a 10 per cent, solution of potassium chromate. The red 
stripe left begins about A 718 and ends abruptly at A 639. 
Yellow fitter . — Dissolve 30 grm. of nickel sulphate in 100 c.cm. of 
water, and in a thickness of 20 mm. this absorbs the red only ; a 10 per 
cent, solution of potassium chromate in 15 mm. thickness absorbs the 
blue, and a 15 mm. thickness of 0*025 grm. of potassium permanganate 
in 100 c.cm. of water absorbs the greeD, leaving A 614 to A 574. 
Breen filter . — Cupric chloride (CuCl 2 , 2H 2 0), 60 grm. dissolved in 
100 c.cm. water, and 20 mm. thickness, passes the green and blue, and a 
* English Mechanic, Dec. 1897, pp. 358-9. 
