TRANSACTIONS OF SECTION A. C75 



graduated exposures for each. Just in front of the film is mounted a positi ve wedo-e 

 of yellow optical glass, backed with an equivalent negative wedge of plain optical 

 glass, the whole having a sliding motion lengthwise. The necessary thickness and 

 length of the wedges are first lound by experiment on the sun, entirely obscured 

 artiBcially by an occulting disc, excepting only the extreme limb. Through a 

 ruby aperture in the camera the observer watches the gradually diminishino- 

 eclipse crescent on the film, and racks the weds'e along, keeping the intensity of 

 the image as nearly as possible constant. The biograph films of the recent 

 eclipse, taken by Mr. J. N. Maskelyne, F.R.A.S., indicate the necessity of this 

 great reduction of the strong light of the crescents, to avoid solarisation ; and show 

 further the ease with which the inner coronal ring can be photographed, Ion"' 

 before and after totality. 



4. On (he ' Square- shouldered' Aspect of Saturn. 

 Bij E. M. Antoniadi, F.R.A.S. 



The author accounts for the abnormal figure assumed by Saturn, under a 

 slight opening of the ring system, by the exUtence of the planet's dark polar cans, 

 checking irradiation along the minor axis of the disc. 



5. On the Types of Sun-spot Disturbances. 

 By Rev. A. L. Cortie, F.R.A.S. 



As an aid to researches connected with sun-spots an attempt has been made 

 to classify them according to some prevailing typical forms. For this purpose the 

 Stonyhurst drawings of the solar surface for the last twenty years have been 

 caretully examined. From these it would follow that spots appear as scattered 

 groups of small spots, as trains of spots, as composite groups consisting of three or 

 more larger spots, as single spots of round and regular outline which may or may 

 not be accompanied by smaller companions, and as single spots of irreo-ular 

 outline at times accomjianied by a train of small companions, or with outliers not 

 arranged in the form of a train. The chief type, however, of which the above- 

 mentioned are in most, probably in all, cases but phases, is the double-spot 

 formation, with a train of smaller spots between tlie two principal spots of the 

 group. In this form the principal spot, which eventually becomes a round spot of 

 regular outline, is generallj' the leading spot ; but in some cases it is the followino- 

 spot, while in other yet rarer cases both the chief spots develop as regular spots'. 

 The mode of development of this leading type is described in detail. The 

 following are suggested as the types which will be probably found to cover every 

 case that may arise. 



Type I. A group of a few scattered small spots. 



Type II. The two-spot formation. 



Type II. a. In which the leader is the principal spot. 



Type II. b. In which the following spot is the principal spot. 



Type II. c. In which both spots are more or less equal. 



Type III. A train of spots. 



Type III. a. With well-defined principal spots. 



Type III. b. Without well-defined principal spots, but mostly penumbra! 

 patches wth scattered irregular umbrje. 



Type IV. Single spots. 



Type IV. a. A single spot of round and i-egular outline. 

 . Type IV. b. A single spot of round and regular outhne, with small com- 

 •panious. 



Type IV. c. A single spot of irregular outline. 

 . Type IV. d. A single spot of irregular outline with a train of small com- 

 punions. 



