1405 
d, 
poe :p becomes about % for the ordinary flint glass, so that: 
dv 
1 dA 1 3) 
nem 
6 dA 
For 4 = 5000 A°, da = 25 A° becomes Te aig 
6. Ribbon-shutter. The shutter which acts periodically and is 
worked electromagnetically, described in I, 4, repeatedly gave cause 
for disappointment, because it was never certain that the light was 
transmitted at the very moment that the cradle passes a chosen 
point of the path. 
This is perfectly certain with the ribbon-shutter, which is dia- 
grammatically represented in fig. 1. 
Fig. 1. 
A band of ribbon Z of lancaster linen is clasped between two 
blocks B, which are firmly fastened at a chosen place of the bed 
of the apparatus. It is passed round the beam with the glass column. 
The beam can execute its usual movement to and fro without being 
hindered by the ribbon, for this can easily slide over the copper 
pieces K, the length of the ribbon remaining constant. There have 
been made two openings in the ribbon of 10 or 15 em., which at 
a certain position of the beam, but only then, allow the light to 
pass through circular holes in the pieces A, and during the time 
that corresponds with the length of the openings in the ribbon. By 
displacing BBS along the bed, the moment at which the light is trans- 
mitted, may be chosen. The edges of the ribbon are provided with 
a hem to obtain greater firmness, and prevent fraying. 
The copper pieces A are smoothly polished, and the friction of 
the ribbon is therefore very slight. Sometimes it was still diminished 
by some tale-powder. 
The electric shutter, which was used in the experiments described 
in II, was now used after a small modification to admit the light 
only in one of the movements to and fro of the beam. For this 
purpose the movable arm is placed before the arc-lamp. The phase 
