NATURAL PHILOSOPHY. 203 



The Committee of the British Association, having the Ivew Observatory 

 in charge, have undertaken, at the suggestion of Sir John Herschel, to se- 

 cure by means of photography a daily record of the appearance of the 

 sun's disk, with a view of ascertaining, by a comparison of the spots upon 

 its surface, their places, sizes and forms, whether any relation can be es- 

 tablished between their variations and other phenomena. The method 

 proposed by Sir John Herschel for accomplishing this object is as fol- 

 lows : 



" The image to be impressed on the paper (or collodionized glass) should 

 be formed, not in the focus of the object-lens, but in that of the eye-lens, 

 drawn out somewhat beyond the proper sitiiation for distinct vision, (and 

 always to the same invariable distance, to insure an equally magnified 

 image on each day.) By this arrangement a considerably magnified image 

 of the sun, and also of any system of wires in the focus of the object-glass, 

 may be thrown upon the focusing- glass ' of a camera box adjusted to the 

 eye- end of the telescope. By employing a system of spider lines parallel 

 and perpendicular to the diurnal motion, and so disposed as to divide the 

 field of vieAv into squares, say of five minutes in the side, the central one 

 crossing the sun's centre, (or rather, as liable to no uncertainty, one of them 

 being a tangent to its lower or upper limb,) the place of each spot on the 

 surface is, ipso facto, mapped down in reference to the parallel and declina- 

 tion circle, and its distance from the border, and its size, measurable on a 

 fixed scale. If large spots are to be photographed specially with a view to 

 the delineation of their forms and changes, a pretty large object-glass will 

 be required, and the whole affair will become a matter of much greater 

 nicety ; but for reading the daily history of the sun, I should imagine a 

 three-inch object-glass would be ample. The representations should, if 

 possible, be taken daily, and the time carefully noted." 



Sir John does not think that a very powerful telescope would be requi- 

 site, but that it should be equatorially mounted, and with a clock-motion 

 in parallel. 



VITRIFICATION OF PHOTOGRAPHIC PICTURES. 



The author of this process, M. Plaut, first procured a photograph, on 

 glass covered with albumen, and subjected it gradually to a strong heat, so 

 as to redden the glass. The albumen was destroyed, and the photograph, 

 if negative, became positive by reflection. The picture was made of pure 

 silver, which adheres quite strongly to the glass, so that it may be polished 

 without alteration. 



On exposing this glass to the action of hydrofluoric acid in vapor, an 

 engraving of the design is obtained over parts not covered by the image 

 formed of the silver. It may also be possible to strengthen the image by 

 a galvanic deposit, and make a kind of plate from which engravings could 

 be taken. 



If, in place of arresting the process at a red heat, it is continued until 



