and Laboratory Methods. 1797 



screwed fast to the board, or cleats may be fastened to the inner face of the 

 board so as to keep the plate from slipping downward or laterally, and a strong 

 button placed on each side will hold it firmly against the board. If the elevation 

 of the optical axis of the porte-lumiere from the floor of the room is less than the 

 distance from the floor to the center of the picture on the screen, it is necessary 

 to incline the plate by bringing its lower edge forward until the base rods point 

 to a spot about six inches below the center of the screen, and then fasten it firmly, 

 as described above. 



The next step is the darkening of the room, and it is much easier and less 

 expensive than many suppose. After successive trials of solid wooden inside 

 shutters, black enameled cloth shades on spring rollers and the same with shal- 

 low boxes constructed around the window frame to intercept the light passing in 

 at the edge of the shade, and finally very dark brown opaque window shade 

 cloth on spring rollers, the latter has been found satisfactory and less troublesome 

 than any of the others. Such an arrangement is most satisfactory on windows 

 having a depth of four, or more inches, between the sash and the inside edge of 

 the window frame. On shallower windows, especially if the color of the window 

 frame is light, the edges of the shades should be boxed in. In all cases the 

 spring roller should be set as close as possible to the sash and the shades should 

 be as wide as the rollers can carry. These directions are not intended to pro- 

 duce absolute darkness in the room, for it is not necessary in order to produce 

 brilliant pictures on the screen. 



Concerning screens it should be observed that a dead white surface is the 

 ideal. A smoothly finished wall with a dead white paint or calcimine surface is 

 best. Next in value as a good reflecting surface is a screen made of heavy opaque 

 white window shade cloth. The white paint used on such a. screen should be 

 zinc white, not white lead, which soon changes to a yellow color. These screens 

 are most conveniently mounted on spring rollers, which should always be made 

 f(3ur inches longer than the shade is wide. Two inches of roller at each side of 

 the shade prevents damage to the fabric if it does not always run true. The 

 common muslin screens are troublesome on account of the difficulty of stretching 

 them smoothly, and the large proportion of light which is lost by being transmit- 

 ted through the' fabric. If, however, the picture is to be observed from behind 

 the screen, this is an advantage. A. H. Cole. 



University of Chicago. 



Permanent preparations of blood may be made by allowing fresh blood to 

 fall drop by drop into a solution of osmic acid (two per cent, acid solution, one 

 part ; one per cent, solution of sodium chloride, two parts ; distilled water, one 

 part). The solution should be constantly stirred while the blood is dropping. 

 Allow the blood and acid to stand one n^ght and then wash the acid away 

 with distilled water. Add alcohol, then clove oil, in which the blood may be 

 kept indefinitely. Before the alcohol is added the nucleus of the corpuscle may 

 be stained in alum-carmine, or the whole corpuscle may be stained in anilin- 

 blue. Mount in balsam. — Microscopicaljounial. 



