When a pure spectrum is examined by a telescope, it is discovered to be intersected by a great number of dark lines, as shown in the following figure, and which are called " Fraunhofer's Lines," he having first dis covered them. There are nearly 400 of these lines.
These lines are produced by the inter ference of undulations of light. In the spectra produced by light from different sources the lines do not occur in the same order ; neither do they occur at the same relative distances when light from the same source is refracted through prisms made of different materials. In the light from the sun and planets the spectral lines occur in the same order ; but in the spectra formed by light from the fixed stars, or by the electric light, or by light produced by the combustion of different substances, the spectral lines do not occur in the same order, nor are they equally numerous.
With respect to the calorific, luminous, and actinic properties of different parts of the solar spectrum ;—It has been shown in the article on light, that light, heat, and actinism are most probably undulations in the same ethereal medium, differing only in the length of the wave ; and it is found by experiment, that both heat, light, and actinism, are capable of producing chemical changes in bodies ; so that the term " ac tinism" merely means that certain sub stances are chemically affected 1?y certain rays (palled the actinic,) residing at a cer tain part of the spectrum.
It is therefore somewhat unscientific to call certain rays " actinic" because they produce chemical changes in certain bodies, and then to say generally that the chemical rays chiefly lie at the violet part of the spectrum, when we know that every part of the spectrum is capable of producing important chemical changes in some substance or other. To say that the heat rays reside mostly at the red end of
the spectrum, luminous rays mostly in the yellow part, and chemical rays mostly in the violet part, is so far unscientific that it is not the statement of a universal law ; for the effects exhibited by the different parts of the spectrum upon a substance placed in it, depend upon the nature of that substance, and are different with different substances. This being the case, we have not included in the fore going fignre of the spectrum the three wave lines of light, heat, and actinism, with which it is generally adorned in popular works on photography.
SPECULtIM METAL. This is generally a compound of about 6 parts copper, 2 parts tin, 1 part arsenic. It is used for metal reflectors. The great reflector of Lord Rosse's telescope is made of 126.4 parts copper and 58.9 parts tin, without the addition of any arsenic.
The word "Brass " was oraitted in the letter B. We may observe in this place, that brass is a compound of copper and zinc, with the addition sometimes of a little lead, tin, and iron. The latter metal shoukl not be introduced in the brass used for philo sophical apparatus. The proportions of the metaLs in brass for turn ing are as follow :— Copper . 61.6 parts.
Zinc . . 35.3 „ Lead . . 2.9 „ Tin . . 0.2 „