The series of observations to be described was undertaken in order to ascertain how far a small pocket spectroscope could be relied upon for the prediction of rain, with a view to its popular use for that purpose.

The instrument employed was Hilger's smallest sized direct-vision spectroscope, its length being one inch and a half, and its diameter half an inch. It was furnished with the ordinary adjustable slit, and had no special provision for the exclusion of dust. It is desirable, however, to use an instrument which has the eyehole protected by a plate of thin glass, and the slit covered with a thicker plate. In this case care must be taken to keep the glass free from scratches, which greatly obscure the spectrum.