REGISTRATION OF THE CHEMICAL ACTION OF TOTAL DAYLIGHT. 615 
the time of insolation does not exceed 2 or 3 seconds, the error introduced by this 
opening and closing may become considerable ; for the purpose of diminishing this error 
by increasing the duration of exposure, the intensity of the acting light is decreased by 
a known amount by allowing the circular disk of blackened metal (fig. 5), out of which 
two segments, each of -^-th of the whole area, have been cut, to revolve rapidly close 
above the upper surface of the slide (A) ; the spindle of the disk, for this purpose, fitting 
into the socket (S, fig. 3) on the stand. As the rate of rotation of the disk does not 
affect the accuracy of the result, it is made to revolve by turning the spindle with the 
hand. In order that the insolation-band carrying the sensitive paper may be made to 
press close against the lower edge of the opening (C), a piece of cartridge paper is placed 
underneath it, having several thicknesses of paper pasted at the part underlying the 
opening, whilst the ends of the same are made fast at the back of the slide. To enable 
the operator to observe when the paper has been sufficiently exposed, a small piece of 
photographically-tinted fixed paper of the requisite degree of shade is gummed upon 
the surface of the permanent paper band so as to lie directly under the opening (C). 
When one observation has been made and the time and duration of the insolation 
noted, the remaining papers can be similarly exposed at any required time, by successively 
bringing them under the central opening (C), the right adjustment being ensured by 
making the corresponding mark coincide with the end of the slide. When all the nine 
papers upon the band have thus been exposed, it can be withdrawn and a second band 
prepared, as the first can be substituted without the necessity of bringing the apparatus 
into a dark room. This is done by means of a small black silk bag or sleeve, open at 
both ends ; one end can be closed round the end of the brass slide by an elastic band, 
and the other is left open to admit the hand. When it is required to withdraw an inso- 
lation-band from the slide, the end of the paper is drawn out into the bag and the band 
rolled up into a small coil, and thus preserved until it can be read off, whilst the new 
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