neous apparatus of a peculiar construction. It consists of a sliding plate with a square 
aperture sufficiently large to permit of the passage of all the rays ; this aperture is 
fitted with a sliding piece, actuated by a screw w’hich projects through and a few inches 
beyond the telescope-tube ; by means of this screw the aperture may be completely 
opened, closed, or reduced to a slit of any required width ; a divided scale being affixed 
to the screw for that purpose. The projecting screw connected with the slide is shown 
in the engraving, on the underside of the tube. 
TOTAL SOLAE ECLIPSE OE JULY 18, 1860. 368 
For sun-pictures, and the photographs of the several phases of the eclipse, the aper- 
ture of the object-glass was reduced to about 2 inches in diameter by means of a stop ; 
but the light of the sun is so extremely powerful, that, even with this small aperture, 
combined with the enlargement of the primary image and its consequent reduction 
in intensity by 64 times, the shortest exposure possible with the ordinary means of 
uncovering and covering the object-glass would be far too long, and would give none 
but solaiized pictures. For this reason the instrument has attached to it an instanta- 
