ANALYSIS BY GREEN GLASSES. 325 (548.) Chemical Spectrum. -Intense action, producing a bronzed line, commences at 60 and extends to 1.10, or over a space equal to 50. A weak action extends down to 45, marking the space occupied by the green ray. A similar modified action extends upward to 1.60. By long exposure a light fringe appears over the space occupied by the yellow rays, the actual chemical spectrum being 1.35 in length. The fringe around the lower part of the spectrum, which is not readily explained, is a tolerably constant result. It may possibly arise from some refraction of the rays near the Fraunhofer line B, within the glass plate. a a (549.) COPPER GREEN of great brilliancy.-The red below the orange cut off; does not shorten the violet end, but produces a great extension of the blue; the green rays encroach considerably on the yellow. The orange occupies the space of 10, the yellow about 16, but these rays blending with the green, this is not easy of exact determination; the green occupying a space equal to 25, and the blue and violet 38. There is a considerable loss of Light in the spectrum. (550.) Chemical Spectrum. - Action commences at 60, and it is continued with full intensity to 1.15. This forms a well-defined dark olive green oval spot; it is somewhat smaller at the upper end; the chemical action is then continued faintly to 1.35, and still more faintly to 1.60. At the lower end the impressed image descends to 50, and a faint border of dusky brown surrounds the spectrum, which is impressed over a space equal 9 a a to 1.5. The principal action is limited to the blue and the rays above it. i (551.) INTENSE GREEN.-All the rays below the orange are cut off; the yellow and green form one tint of pale pea-green. The blue rays are very light in colour, losing but little of their illuminating power, and these are fringed with a deep band of indigo; no violet rays apparent. (552.) Chemical Spectrum. The action here on the most sensitive collodion plates is exceed ingly slow, and after an exposure of five minutes in the brightest sunshine, the only indication of any chemical action is the appearance of a faint spot near the line H. This when acted on by the pyrogallic acid becomes very dark, and another spot a little beyond the violet rays makes its appearance. a' In the camera obscura, which has been devised for working with the very sensitive and beautiful collodion process in the open air, yellow glasses have been introduced for the purpose, as it was thought, of cutting off the chemical rays, at the same time as Light enough was admitted to enable the operator to see his work. The results obtained clearly prove that rays, chemically active for collodion, pass the yellow media very freely; some green glasses, as the above, offer much more obstruction and were consequently recommended; but more recent experiments have shown that the ruby-red glasses are far more effective than any others. a (553.) DEEP IRON-GREEN.-Cuts off the lower red rays; admits the permeation of the orange rays freely. The green very much blended with the yellow, so that it is only by adjusting with great care that a line of yellow can be seen. The blue and violet rays suffer scarcely any change, the lengths of these rays being relatively as follows:-green 30, blue 25, violet 15. ABSORPTIVE ACTION OF BLUE GLASSES. 327 (554.) Chemical Spectrum. The action of this spectrum is very slow, and confined to the limits between the mean green ray and the extreme violet. In a great many experiments the spectrum impressed has been always limited to the space 50; that is, it has commenced in the middle of the luminous spectrum and terminated with the violet rays. Very weak hyposulphite of soda washes off the darkened portion so readily, that I am led to infer that it is an exceedingly superficial dust upon the surface only. a (555.) DEEP IRON-GREEN. NO. 2.-All the reds of flowers observed through this glass become nearly black. The spectrum exhibits two spots of pure crimson; perfect blackness between them. A spot of yellow of great purity, from which the green shades off into a light blue, which becomes very bright, and then passes into a line of indigo. The violet is entirely wanting. (556.) Chemical Spectrum. - By long-continued action there is scarcely a trace of any chemical change. Here we have an example of a spectrum in which the blue class of rays, ordinarily called the chemical rays, are very brilliant, yet they are chemically inactive upon this most sensitive photographic preparation. (D.) Series of Blue Glasses. b a (557.) DEEP COBALT-BLUE. — The red of flowers, as of the Fuchsia, &c., is entirely lost, and not to be distinguished from the green leaves. The surface of leaves appears a grey or blue, with a few exceptions; the Arbor vitæ assumes a red-brown colour, remarkable from the striking contrast it makes with the surrounding trees. The leaves of the Currant and several other plants appear red on their under surface when examined through this glass, the light falling on the upper surface, and being transmitted. Yellow Nas y a Blue Larkspurs not The violet and blue turtiums become of an intense brown. to be distinguished from the leaves. rays form a large oval, which, encroaching on the green, reduces it to a line bordering the lower edge of the blue. Yellow, a well-defined circular spot, ordinary red obliterated, and the extreme red forming a well-defined circular image quite surrounded by a black band. A prism of crown-glass gives the same result, as does also the hollow prism filled with castor oil. The extension of this spectrum is remarkable. (558.) Chemical Spectrum.--The rapidity of action with this medium prevents the marking of many of the more remarkable gradations of change which appear to go on within the chemical spectrum. Chemical action commences between 70 and 1.70, and extends up to 2.20. The inner portion of this spectrum appears the lightest, but in reality the action has been much more intense over this section than on any other part, and the semitransparency of this portion is due to the complete production of metallic silver in a state of fine division. Ammonia will dissolve off the outer dark-brown edge, but does not act in the slightest degree on the inside oval space. The spectrum obtained without any interposed medium is, upon collodion, of less extent than that now described; which is, usually, a long flame-shaped band of 1.60 or 1.70 in length, and of uniform intensity throughout. (559.) COMBINED BLUE AND GREEN GLASSES.-Looking at the spectrum through these glasses, every trace of red is obliterated, the resulting spectrum being a pure green and blue. Making the rays pass from the prism to a lens, and causing the concentrated rays to permeate this combination, the result is somewhat more decided. The spectrum is not shortened at the most refrangible end, COMBINED BLUE AND GREEN GLASSES. 329 but the red of the violet is entirely removed, forming a a b pure blue patch equal to 8. Over the space marked g the green is far more luminous than over any other part, and the rays gradually darken towards the lower end. (560.) Chemical Spectrum. -The spectrum impressed by long exposure commences at 45 and terminates sharply at 10, there being no indication of any action beyond the visible spectrum. It will be found by examining the drawing of the luminous spectrum obtained when the Light has passed this combination of glasses, that the action commences at the lower edge of the green rays. The whole space impressed has equal intensity throughout, with a brightening of the silver in the middle. (561.) A LIGHT BLUE.-The spectrum is represented in its three primary rays, suffering a little reduction in length; when concentrated by a lens a little violet appears at the extreme edge of the blue. Natural objects do not suffer much change when observed through this glass; purple flowers lose more of their red than of their blue, and violet-coloured ones appear nearly pure blue. (562.) Chemical Spectrum. The action commences at the lower edge of the blue rays. 55 from 0, and extends to 1.40, when it is suddenly interrupted. Considering the usual character of blue glasses, and that this one is of an unusually transparent nature, it will be necessary to subject it to a much more searching examination than it has yet received. That D a a' a' |
