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spaces between the filament images on the shutter are filled with the reflected light (Fig. VII. B.).
This setting is very important. If the mirror image falls on the lamp filament coils instead of between them, not only is this additional light cut off from the screen, but the life of the lamp may be slightly shortened due to overheating. Whether the mirror is properly set or not may easily be observed by quickly inserting and removing any opaque screen between the lamp and the mirror, noting the effect on the image on the shutter. If desired, the focusing of the lamp and mirror may also be accomplished on the screen instead of the shutter by removing the objective lens. The appearance of the screen with or without the mirror images properly set will then be as in Fig. IX. A. and B. This method of focusing results in slightly more even distribution of light on the screen with very little decrease in average intensity. The degree of unevenness of screen illumination, when the filament is focused on the shutter is, however, not discernible to the average eye. Tests to determine the effect on total screen illumination (neglecting the question of evenness or otherwise of distribution) of having the mirror improperly set, gave the following results:
Mirror properly located Relative light on Screen (Lumens) loo
88. o
" 74-5
74-0
74-0 '' 74
90
i-io''
to one
side
" "
< ( < (
( ( <
< < < (
2-10''
(( ( (
( < <
( ( ( (
3-10''
(( ((
<< (
" 4-I0''
5-10''
a a
" 4
a a
i-io''
ahead
<< ((
2-10''
<<
a a
3-10''
<<
it ii
<(
11 li
*' 4-10''
5-10"
"
a a
i-io''
above
" "
2-10''
(<
a ((
3-10''
(<
<( ((
" 4-10"
5-10''
**
<( a
79 76 73 84 68 68 68 68
A possible source of large loss is in the alignment of the condenser lens and the objective lens. It is important that the face of the condenser be parallel to the face of the projection lens, and also that the center of the two are on the same axis passing through the center of the light source and the center of the mirror. Tests were made in which the condenser was properly lined up and the screen lumens measured. The condenser was then moved to one side in steps of i-io'' also up in steps of i-io''. The results were as follows:
Condenser properly centered Relative light on Screen (Lumens) 100
I-IO''
to
one si
" 2-10"
"
a (
" 3-10"
"
t( (
4-10"
<<
iC <
li "
72.5
a
39-5
i( a
24.0
n n
20.5
li a
20.0
58