Fasten the lower strip of the two permanently to the disk, leaving the other strip free to move in an horizontal plane about the center of the disk. You will then have a stand like that represented in Fig. 3,

A being the stationary arm, and B the movable arm. Make the small disk of wood also into a table (see Fig. 4) by fastening in three sharp—feet screws will do. Place this smaller table centrally on the larger Fig. 4. one, and it becomes the platform on which the prism is to stand. The next thing is to put the collimator and telescope in their places. For this purpose two small oblong blocks of wood are needed, each one having the upper part hollowed out into a groove to take—one the collimator, the other the telescope. (See Fig. 5, d, d.) The block carrying the collimator is placed on the stationary arm, and the one with the telescope on the arm which is free to move. Both are secured in their places by elastic bands. The blocks of wood must necessarily be of such a height that the axis of their respective tubes (collimator and telescope) may be brought into alignment (1) with

each other, and (2) with the prism. The instrument is then complete (see Fig. 5), where A is the collimator, B the prism, and C the telescope.

The manner of using the spectroscope has been described in such works as those I have already mentioned, and does not properly belong to an article such as this, but perhaps a few hints as to the adjustment of the instrument may not come amiss to the beginner. The