O. N. Rood on Photometric Eaperimenis. 147 
Second. All those portions of the ground surrounding and 
in contact with the “spot,” must be equally illuminated, and 
the texture of the ground must likewise be uniform. 
urd. The. transition from the ground to the “spot” must 
be perfectly sharp and sudden, so that not the faintest border 
can be seen surrounding it. 
Fourth. Tf it be required to render the invisibility of the 
“spot” more than momentary, it is equally essential that the 
ratio subsisting between the two sources of illumination should 
be truly constant. ; 
If either of the first three precautions be neglected, the dis- 
appearance of the spot becomes entirely impossible, while it is 
only by a peculiar arrangement of apparatus that the fourth can 
actually be realized. (See second part of this article.) 
Screen.—A plate of colorless glass of good quality is coated 
with photographic collodion and immersed for a few minutes in 
2 solution of nitrate of silver, (“ bath,”) as though it was the in- 
above mentioned dimensions it becomes an annoying object for 
observation, while if it be larger, it is difficult to illuminate it 
1. ee 
L 
HE 
ieee oes 
B , st woe 
uniformly. This plate is seen at P, in figure 1, the collodion 
side bei - suited coated she eye of the observer, and the other 
except just opposite the spot, covered by a coating of oon 4 
‘Slack mixed with weak shellac varnish, pedis 1 
Ron-reflecting surface. At the distance of an inc 
collodion plate, there is fastened, parallel to it, a plate of color] 
: 
te 
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