106 
ASTRONOMY: HALE AND ELLERMAN 
shown that stereoscopic vision can be obtained when this angle is con- 
siderably smaller. Enlargement of the image by a telescope, and the 
effect of increased inter-ocular distance realized by combining two 
photographs of a rotating object like the sun, permit the angle to be 
reduced to surprisingly small values. Thus even on Helmholtz's basis 
two photographs of hydrogen flocculi on the central meridian of the sun, 
taken only five minutes apart at the focus of the 60-foot tower telescope, 
can be combined to give stereoscopic rehef. At a distance of 45° from 
the central meridian this interval must be increased to nearly 7 minutes, 
but further magnification of the image can be utilized to reduce the 
minimum time for this region to five minutes or even less. A short 
time interval serves very well near the Hmb, in spite of the smaller dis- 
placement, because of the enhanced effect of relief due to the curva- 
ture of the sun. 
The accompanying stereoscopic picture represents the hydrogen 
flocculi above and surrounding a southern group of sun-spots about 
45° west of the central meridian on August 7, 1915. The two exposures 
were made at 6h. 19m. and 6h. 26m. a.m., respectively, and were thus 
separated by a time interval of 7 minutes. The enlargement from the 
original negative is 1.4 diameters, corresponding to a solar image about 
24 cm. in diameter. The conditions necessary to secure stereoscopic 
vision are thus fully realized. It should be noted, however, that while 
the appearance of rehef is probably genuine, so far as the chief elements 
of structure are concerned, certain false effects are present due to 
slight distortion of the images. These have the appearance of a hori- 
zontal depression running centrally across the picture, between two 
rounded ridges, on which lie the fringes of distended flocculi above 
and below the axis of the spot-group. Other false effects may arise 
in the case of individual details which change materiafly in form be- 
tween the exposures. These are few in number, however, and may be 
readily detected. 
In examining the photograph, one is struck by the general resemblance 
to Langley's drawings of sun-spots and the photosphere (fig. 2). While 
the difference in scale between the minute phenomena depicted by 
Langley and the coarser details of the higher atmosphere must be borne 
in mind, the resemblance can hardly be devoid of significance. As we 
have already seen, the minute hydrogen floccuh, in undisturbed regions 
away from spots, are granular m appearance, though larger than the 
minute grains of the photosphere. At some distance from the spot 
group (near the margin of the picture) they give place to slender fila- 
ments, extending toward the axis of the group as the much finer penum- 
