176 On an Improved Method of 



stances, it would be best to start tbe shutter from a greater height, 

 which would give greater velocity to the passage of the slit, and 

 any available fraction of time desired might thus conveniently be 

 obtained. The whole arrangement is inexpensive, and may be 

 manufactured in a few hours by anyone, out of a deal board, a few 

 pieces of cardboard, and a yard or two of cotton velvet. 



Of course the fractional measures of time obtained in this way 

 are not absolute, since the friction must be variable, unless the 

 apparatus were made in a more costly manner of metal. But I 

 have found that the variations thus introduced are so small that 

 they may be disregarded, and that while the starting-point remains 

 the same, the width of the slit in the falling shutter indicates frac- 

 tions of time which may confidently be counted upon to give pro- 

 portional photographic results. 



The next subject for remark is the arrangement employed when 

 the heliostat is dispensed ^^'ith. 



For this pm-pose the contrivance usually employed for the solar 

 microscope answers very well. A circular disk of brass, ^^"ith 

 toothed edges, is let into a square plate of the same metal, and is 

 turned by a small toothed wheel, to which a suitable button or 

 milled head is attached. Through the centre of the disk passes a 

 tube six or eight inches long and two inches in diameter, the outer 

 extremity of which is fitted to receive the large condensing lens. 

 Just below this tube an arm is fii-mly attached to the outer surface 

 of the disk for the purpose of carrjdng the mirror or right-angled 

 prism, to which any deshed inclination can be given by a rod passing 

 through the disk by the side of tlie tube. The whole arrangement 

 is quite like the similar parts of the ordinary solar microscope, and 

 hence needs no minute description ; it is fitted into a window- 

 shutter, which must of course face to the south, and the room being 

 darkened the motions of the mirror, or prism can readily be con- 

 trolled from within. If the condensing lens is used I do not think 

 any material advantage can be obtained from the prism, and its 

 expense is a decided objection. In the winter season in this latitude 

 a prism of over five inches hypothenuse is required, and its cost is 

 a serious item. An ordinary glass mirror answers, I think, quite 

 as well for the tissues and most other purposes. There are, how- 

 ever, certain objects, such as the Plem'osigmata and some other 

 diatoms, the Nobert's Test-plate, and the scales of certain insects, 

 for which the condensing lens is unnecessary. The achromatic 

 condenser, illuminated by a parallel solar pencil, answers better in 

 these cases, and if it is properly managed no diffraction or inter- 

 ference phenomena are produced. I am satisfied that in such cases 

 the pure parallel pencil obtained from the prism gives better defini- 

 tion to the image than can be obtained by the double pencil reflected 

 from an ordinary glass mirror. A mirror silvered on the reflecting 



