212 Mr. A. Ckud£t on 'tlwOpties of Photography. 



which they chiefly originate lie, in the -northern hemisphere, 

 nearest the region (long*. 253°, IN. lat. 57°) from which the elec- 

 tric and material currents radiate, and because in the northern 

 hemisphere alone is the flow of the material currents in the 

 direction to be attended with a demagnetization. Again, the 

 spots are confined to a narrower belt in the northern than in the 

 southern hemisphere, because, the magnetic intensity of the 

 northern hemisphere being the greatest (p. 206), the ordinary 

 limiting parallel of the spots (which is the circle at which the 

 demagnetizing action from either of the two causes specified, 

 together with the electric discharges along the photospheric 

 columns, cease to be sufficient to effect the dispersion of these 

 columns) must lie nearest the equator in that hemisphere. 



It -is to be observed that spots may arise from electric dis- 

 charges along the photospheric lines of polarization, although 

 no demagnetizing action may come into operation; and it is even 

 possible that indirectly an increase of magnetic intensity may, 

 in special cases, cooperate with such currents. 



Note. — Since the foregoing was written, the line of investigation 

 here entered upon has been followed up, and new and important ge- 

 neral results obtained. One of the principal results is, that the density 

 or quantity of matter of the sun s photosphere, in the region of the spots , 

 experiences periodical augmentations, in consequence of certain effects 

 produced by Jupiter and Saturn while, passing by the first quadrant 

 of heliocentric longitude and the sun's north magnetic pole, and 

 that these augmentations of density, in connexion with the subse- 

 quent diminutions, are one of the determining causes of the great 

 variations that occur in the length of the period of the sun's spots 

 (viz. from eight to sixteen years). Another cooperative cause 

 consists in, the diverse positions of the planets, especially of Venus, 

 with respect to positions of favourable action, at the epochs of helio- 

 centric conjunction with the earth. The other causes have been in- 

 timated. 



[To be continued.] 



XXVII. Optics of Photography. — On a new Process for Equali- 

 zing the Definition of all the Planes of a Solid Figure represented 

 in a Photographic Picture. By A. Claude t, F.R. S* 



ONE of the greatest deficiencies of photography in the repre- 

 sentation of solid figures is the impossibility of obtaining 

 a well-defined image of all the various parts situated on different 

 planes; for it is well known that the best object-glasses can 

 give a sharp image only for the plane in focus; the images of 

 the objects situated before and behind are more and more con- 

 fused as they are more and more distant from that plane. 



* Communicated by the Author, having been read at the British Asso- 

 ciation, Nottingham Meeting, 1866. 



