Histological Preparations by Sunlight. 265 
4 
| venient velocity is attained for a magnifying power of two to 
five hundred diameters, arranged as I have described. For still 
“= shorter exposures, necessitated by lower powers or other cireum- 
| _ Stanees, it would be best to start the 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 con- 
| Yeniently be obtained. The whole arrangement is inexpensive, 
d may be manufactured in a few hours by any one, out of a 
deal board, a few pieces of card-board and a yard or two of 
Cotton velvet. Seo 
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 meta 
But I have found that the variations thus introduced are so 
small that they may be disregarded, and that while the start- 
Ing point remains the same, the width of the slit in the falling 
‘Shutter indicates fractions of time which may confidently 
Counted upon to give proportional photographic results. 
€ next subject for remark is the arrangement employed 
When the heliostat is dispensed with. 
or this purpose the contrivance usually employed for the 
A circular dise of brass, 
: is turned by a small toothed wheel, to which a suitable 
: button or milled head is attached. Through the center of the 
carrying the mirror or right-angled prism, to w. 
Melnation can be given iy a rod passing through the ese by 
the side of the tube. The whole arrangement is vaio : _ 
© motions of the mirror or prism can 
Jour. Scr.—Turrp Serres, Vou. I, No. 10.—Oct., 1871. 
138 
