1865.] Prof. Phillips on the Surface of the Sun. 477 



which under the general title of " porosity " have been recognized over 

 the whole face of the sun. Descriptions cannot be complete, but, what is 

 more, they may be, and probably often are, misleading words which call 

 up right ideas of things often fail very much when required to perform 

 the same function for new objects not well understood. With this con- 

 viction in my mind, I have requested the Royal Society to accept a few 

 drawings representing features on the sun's face as they appear to me 

 looking through a telescope of known dimensions, and used in a certain 

 way. If observers would send sketches made at the telescope, showing 

 what they see, or think they see, not finished paintings to illustrate 

 hypothetical ideas, these sketches, by gradual accumulation and com- 

 parison, would at last furnish evidence by which even a great theory might 

 be brought to a satisfactory test. Therefore it is that I presume to offer 

 to the Royal Society some additional sketches of the "porosity" of the 

 sun, as seen in good observing weather in this month of October*. 



The whole surface of the sun, as seen on the 24th and 25th, appeared 

 quite free from any dark patches large enough to be called spots offering 

 in this respect a singular contrast with its aspect on the 1 7th, when the 

 large doubly nucleated spot, of which I sent a sketch a few days since, 

 was so conspicuous near the (apparent) right edgef. 



On this apparently even and marble-like surface, a power of 50, with 

 the full aperture of 6 inches, made manifest the existence of the porosity 

 at every point, from the centre to near the edge, the distinctness being 

 greatest over all the middle part of the area. By applying successively 

 powers of 75, 100, 135, and 180, it was easy to observe the general effect, 

 and the particular features of diversity. The clock-rate being regulated 

 exactly, any particular part of the disk might be kept continually under 

 view ; and to increase the distinctness of the object, or rather the comfort 

 of the observer in looking at it, the field was contracted by diaphragms to 

 one-half or one-third of the usual diameter. The great obstacle to a strict 

 observation of any small selected part of the sun's disk is unsteadiness of 

 the head, a circumstance troublesome to portrait-photographers, but more 

 injurious to astronomers. I believe this kind of error to be one of the 

 elements of personal equation, and that it can sometimes only be cured by 

 allowing the observer to take hold of the moving telescope. This, thanks 

 to Mr. Cooke's solid construction, can be safely done. 



The sketches now presented relate only to the appearances presented to 

 one observer, with the precautions stated ; to what degree they are affected 

 by " personal equation" remains to be proved by comparison with others, 

 and I hope better drawings. 



* Drawings made on a former occasion, and presented to the Royal Society, may be 

 referred to for comparison. (See Plate XII. fig. 6.) 



f Telescope furnished with diagonal glass, the rays reflected to the West. By this 

 arrangement the usual reversal of the object in every direction becomes limited to the 

 vertical direction. 



2o2 



