other at the present time (and which, by-thebye, was invented by Boeckmann nine years before Dr. Mason was born), together with a great many others constructed of animal, vegetable, and mineral substances. Of all these, not one has the power of indicating the state of atmospheric moisture for more than a few yards around the instrument; but there is one that will inform you of the state of invisible moisture of a section of air many miles in thickness, the penetration depending on the amount of visible moisture present at the time of observation.

The following classification suggested by Pouillet is not perfect, but will do for all ordinary purposes. He divides them into : 1st. Hygrometers* (or hygroscopes †) of absorption.

2nd. Hygrometers of condensation, i.e., dew point instruments.

* vypós (hugros), damp, and μerpov (metron), measure. typás (hugros), damp, and oxoréw (skopeo), to look at.

3rd. Hygrometers of evaporation, i.e., dry and wet bulb thermometers.

4th. Chemical hygrometers, for determining the amount of vapour by analysis. And I think we might now add to these the hygro-spectroscope, or rainband spectroscope.

During the last few years this new form of hygrometer has been rapidly advancing in popular favour; but a want has been felt by many an observer, which it will be my endeavour, in the following pages, to supply in as brief, plainly worded, and exactly described manner, as will tend to assist the tyro in his preliminary difficulties, with the manipulation and deductions therefrom, of an instrument of such fine and delicate research.

There are two spectroscopes at present in use, both of moderate dispersion; one, called 'The Rainband Spectroscope,' is an excellent little instrument, very portable, and to a

The other, named 'Grace's Spectroscope,' now becoming the favourite, is larger, but with the same amount of dispersive power; the increased size of the spectrum in it is of

FIG. 2.-Spectroscope with attached Micrometer.

great utility in clearly defining lines and bands, besides the advantage of being able to use it earlier and later in the day than the former kind, on account of the greater amount of light admitted.

In several other respects it differs from the first; it has a milled wheel, with lever for

adjusting focus, and instead of plain glass nearest the eye, a concave lens is fitted.

Mr. Browning, the eminent optician in the Strand, has fixed to my Grace's an adjustable photographic micrometer, with prism of comparison. This combination makes the perfection of an instrument for meteorological purposes.

It would be as well now to proceed to a

description of the Rainband Spectroscope,' which, with the above exceptions, will equally apply to the other instrument.

It is composed of two cylinders, one fixed and the other movable, the latter sliding within the former; at the further end of the fixed portion, which is 3 inches in length, the aperture is covered with a cir

cular piece of microscopical glass, and just within this are two parallel jaws, adjustable by means of a milled wheel outside, to admit more or less light, according to the requirements at the time of observation. The movable part, 2 inches long, has at the extreme end a collimating lens to collect the light which passes through the slit, and to throw it in parallel rays upon the prisms placed

FIG. 4.-Section of Direct Vision Spectroscope.

behind it. These prisms are five in number : one, three, and five are made of crown-glass, two and four of flint-glass. (Vide Fig. 4.) They are all cemented together, and the effect of this combination is to give an image of the prismatic spectrum of sufficient dispersion to show the position of the dark lines, both constant or solar, and variable or telluric-that is, due to vapour in our atmosphere.