644 



NATURE 



\Oct. 17, 1878 



The simple and beautiful contrivance by which M. 

 Janssen has succeeded in getting rid of irradiation, is to 

 restrict the photographic action to one small sheaf of 

 rays of the spectrum, viz., those which extend from the 

 line G a short distance towards H. In a series of experi- 

 mental photographs of the spectrum which M. Janssen 

 took in my presence (with calc-spar prisms and a. rock- 

 crystal lens) and in which the time of exposure was varied 

 successively from two-thirds of a second to three minutes, 

 those obtained with the shortest exposure represented 

 only that part of the spectrum immediately contiguous to 

 &and extending a short distance towards H ; and in this, 

 the spectral lines were exhibited with extreme sharpness. 

 With a more prolonged exposure the range of the image 

 was greatly extended in both directions, accompanied by 

 intensified action in the G-H region, which impaired the 

 local definition. I Taking advantage of this fact, in the 

 new process, the collodionised plate is exposed to the 

 sun's action only so long as to allow of the action of the 

 most actively photographic rays, and this is the cardinal 

 condition of success. In practice, the duration of the 

 exposure is restricted to between ^q\j^ and 5^^^^ of a 

 second, in summer, being varied according to the season 

 and to the time of day. The means by which this deli- 

 cate adjustment is effected and verified will be described 

 presently. 



A second condition is, so to adjust the distance of the 

 sensitised surface from the lens of the instrument, that it 

 shall exactly coincide with the focus of the G rays. The 

 necessity of this precaution will be readily understood 



when it is borne in mind that no lens is perfectly achro- 

 matic, and that, in virtue of the first condition, the rays 

 in the vicinity of G alone produce the image. The re- 

 maining conditions are, the adoption of as large a plate 

 as can be readily manipulated and some improvements 

 in the process of preparing and developing the plates, 

 whereby a very perfect surface is insured for the recep- 

 tion of the image, and a graduated development after 

 exposure. 1 



The photoheliograph employed was constructed spe- 

 cially for the Meudon Observatory by M. Prazmouski 

 of Paris. It has an object-glass of five inches diameter, 

 and a reversing ocular, giving an erect image on the 

 sensitized plate. The finding telescope casts an image 

 on a disk of ground glass ; by observing which, the ope- 

 rator can judge the exact instant for releasing the sHding 

 screen, which causes the instantaneous exposure of the 

 sensitised plate. At present, the position of the telescope 

 is adjusted by means of winches worked by hand. The 

 construction of the sliding screen, on the accurate adjust- 

 ment and working of which the success of the operation 

 mainly depends, will be understood on reference to the ac- 

 companying rough diagram (which, however, is not drawn 

 to scale, and in which, to avoid confusion, some of the 

 minor details are omitted), aa is an oblong brass plate 

 which serves as a frame for the mechanism, and is intro- 

 duced through a slit in tl^e side of the telescope, exactly 

 at the spot where the real image is formed by the object- 

 glass. At the spot where the image falls, the plate is 

 pierced with a circular aperture somewhat larger than 



i...'-^'2u_. 





.^.J A 



the image, shown by the dotted circle C. But this is 

 completely covered by the sliding screen B, excepting 

 such portion as is momentarily uncovered by the trans- 

 verse slit D, in its passage across the image. The width 

 of the si t D can be varied by means of a micrometer 

 screw, which is omitted in the diagram. The sliding 

 screen works between four small grooved wheels ee, fixed 

 to curved springs, which press them against the edges of 

 the screen-plate ; and one of these edges is shaped in the 

 manner shown in the diagram, so that the pressure is 

 increased from the instant at which the slit D reaches 

 the margin of the circular aperture c, neutralising the 

 acceleration of the movement by the continued action of 

 the springs, and rendering it uniform during the passage 

 of the slit across the image.^ Thus the image is allowed i 

 to fall on the sensitised plate, not as a whole, but in ' 

 successive slices, and the width of the slit is so adjusted I 

 to the rate of motion, that each slice is exposed during j 

 from TT^Vo- to Tj^Vtr of a second only. The motion of the 

 screen is eftected by three spiral springs, two of which 

 s s, are shown in the diagram (the central spring being 

 omitted). The fixed ends are attached to a bar bb 

 screwed to the slide A A ; the free ends to a stud on the 

 bar / which projects from the proximal end of B, is bent 

 twice at right-angles and terminates in the hook k. In 

 setting the screen before operating, a loop of twine is 



\ Ow.ng probably to spherical aberration. 



"^ this, M. Janssen suggests, may be ctberwise effected bj' an arrange- 

 ment wliich will arrest the action of the springs at the instant when the slit 

 reaches the margin of c. 



passed over the hook, which is then drawn towards the 

 clip c, extending the springs s s, and bringing the screen 

 into the position shown by a dotted line on the diagram. 

 The twine is made fast in the clip c and all is in readiness 

 for the operation. At the critical moment, the retaining 

 string is cut and the slit D is rapidly drawn by the tension 

 of the springs across the field, till checked by a stop not 

 shown in the diagram. The movement of the screen is 

 generally horizontal, but a gravity compensation is 

 attached which can be employed when the movement is 

 vertical. 



The rate of the movement and the uniformity of the 

 motion are determined by attaching, by means of wax to 

 any part of the sliding screen, a glass slip coated with 

 lamp-black. A tuning fork with an attached bristle, being 

 made to vibrate transversely to the movement of the 

 screen, the latter is released as in the actual operation of 

 photographing, and the length of the ware marked by the 

 bristle on the carbonised surface, multiplied by the width 

 of the slit D, and divided by the number of vibrations of 

 the fork per second, gives the duration of the exposure ; 

 while the uniformity of the movement is tested by the 

 equality of the wave-lengths, which correspond to the 

 passage of the slit D across the circular aperture C By 

 this means, the uniformity of exposure can be regulated 

 to Tiroin) of a second. 



In order to obtain the exact position of the sun's axis 

 on the plate, the instant of the exposure is noted by the 



^ In th'.s part of the process, M. Janssen acknowledges the assistance of 

 M. Arents, to whom the writer is also indebted for much information. 



