ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
145 
corresponding to thirteen different arrangements of this grating. The 
black oblongs represent bright spectra, the hatched ones those with 
weakened intensity. 
For the purpose of investigating to what extent the peculiar 
character of an abnormally diffracting grating has influence on the 
observed diffraction phenomena, eight photographic gratings on silver 
bromide gelatin plates were prepared. These gratings had the follow- 
ing dimensions in millimetres : — 
Grating. 
Width of clear space. 
Width of opaque band. 
I 
0'07 
0-158,0-035 
II 
0-175 
0-158 
Ill 
0-298 
0-035 
IV 
0-084 
0-084 
V 
.. 0-158,0-035 
0-07 
VI 
0-14 
0-193 
VII 
0-114 
0 053 
VIII 
.. 0-07, 0-035 
0-28, 0-035 
Their arrangement may be better understood from fig. 23, where 
they are represented under a magnification of twenty. 
Grating I. is an attempt to represent the cross striation of a 
Hydrophilus magnified about 48 times. Nos. II. and III. are produced 
from No. I. by removal in the one case of the narrow, and in the other 
Fig. 23. 
■ ■ ■ ■ li 
I I I I 
mum 
v| II II || || 
■ ■ ■ I 
wl 1 1 1 1 1 1 1 1 
"■III ■III! 
of the broad dark bands. No. IY. is a simple grating, with spaces and 
bands the mean of those of No. I. No. V. is a negative of No. I. In 
No. VI. the width of the dark bands corresponds to the sum of the 
width of a broad and narrow band, and the width of a space to double 
the width of a space of grating I. In No. VII. the distance between 
the equally wide spaces is equal to the mean distance between the 
spaces in I. Lastly, grating VIII. is an attempt to represent a 
Hylobius muscle with striaj Q, N, and Z. 
The diffraction phenomena produced by these gratings were observed 
in a dark room. The distance between the spectra was measured by 
means of a scale 10 cm. long suspended just above the aperture of the 
diaphragm in front of the zircon thread. This scale was illuminated 
from the side by an argand burner, and had white and black divisions 
alternately at distances of 1/4 cm. The gratings were placed at a 
distance of 150 cm. from the source of light, and the intervals between 
1892. L 
