ASTRONOMY: HALE AND ELLERMAN 
103 
pare the structure thus revealed at various levels in the solar atmosphere 
with that of the lower-lying photosphere and sun-spots. 
Langley's well-known paper^ 'On the Minute Structure of the Solar 
Photosphere/ illustrated with his unrivalled drawings, is still our best 
source of information regarding the structural details of the photosphere 
and sun-spots. 2 After referring to the spots and faculae, Langley goes 
on to say: 
On attentive examination it is further seen that the surface of the sun 
everywhere .... is not absolutely uniform, but is made up of fleecy 
clouds, whose outhnes are all but indistinguishable Under 
high powers used in favorable moments, the surface of any one of the fleecy 
patches is resolved into a congeries of small, intensely bright bodies, ir- 
regularly distributed, which seem to be suspended in a comparatively dark 
medium 
These bright bodies, called 'rice-grains' by Stone, were found by 
Langley to average from V A to 2" .6 between centers, the distance de- 
creasing with increase of telescopic power. 
"In moments of rarest definition I have resolved these 'rice-grains' 
into minuter components, sensibly round, which are seen singly as points 
of light, and whose aggregation produces the 'rice-grain' structure. 
These minutest bodies, which I will call 'granules,' it will appear subse- 
quently can hardly equal 0".3 in diameter, and are probably less." In 
the two squares near the lower right-hand corner of the drawing (fig. 
2, Plate I), the granules are the minute bodies forming the not very 
definite clusters or 'rice-grains.' The squares are \ V.6, corresponding 
nearly to 5000 miles, on a side. From a careful estimate of the size and 
number of the granules and 'rice-grains,' Langley concludes that "the 
properly luminous area is less than one-fifth of the solar surface."'^ 
The sun-spots shown in the same figure combine details from several 
different spots, but accurately indicate the general character of the 
structure, which is described by Langley as follows: 
"The penumbra is all but wholly made up, as it appears on a first 
examination, of cloud-forms whose structure makes them seem like 
fagots or sheaves of some elongated objects." Under the highest powers 
"the penumbra is resolved into 'filaments' of extreme tenuity, which 
by their aggregation make the 'thatch' (a term used by Dawes), just as 
the minute granules of the photosphere compose the 'rice-grains'." 
"It seems to me that there is no room for doubt, that 
'filaments' and 'granules' are names for different aspects of the same 
thing, that filaments in reality are floating vertically all over the sun, 
