516 



NOTES AND QUERIES. 



[Dec. 29. 1855. 



fell in 1720 ; blowing them to atoms, and scattering the 

 materials like cliafF. The east spire of the convent of 

 Dissentis was thrown down by the gust of an avalanche, 

 ■which fell more than a quarter of a mile off." — P. Ixvi. 



I conclude with an anecdote of Oliver Cromwell, 

 who seems to have had a narrow escape at the 

 battle of Marston Moor. How far he thought 

 himself affected by the loind of the hall, I do not 

 know ; at all events, he seems to have been more 

 frightened than hurt : 



" Here it was that, from their invincible bravery, his 

 troopers obtained the well-known name of Ironsides. At 

 the first play of the artillery, their leader had a narrow 

 escape from a cannon-ball, which almost grazed his head. 

 Those who were near him imagined, for the moment, that 

 he had been killed; but, instantly recovering his self- 

 possession, he remarked, smilingly, that " a miss was as 

 good as a mile! " — Memoirs of the Court of England, See, 

 by J. H. Jesse, vol. iii. p. 36. 



WiLXTAM Bates. 



Birmingham. 



PHOTOGRAPHIC CORRESPONDENCE. 



On a Concave Field for Photographic Pictures. — Being 

 an ardent admirer, though to a very limited extent a 

 practitioner, of the fascinating and important art of 

 photography, I have long thought it well deserving any 

 consideration and study which may tend to advance it 

 towards perfection ; and with this view, I beg to send a 

 description of a contrivance which may, perhaps, be con- 

 sidered as of some importance. 



All photographs hitherto produced, notwithstanding 

 their general beauty and excellence, are yet defective in 

 one particular, viz. the comparative indistinctness at the 

 margins of the picture. This arises from the spherical 

 distortion of the lenses, by reason of which the foci of the 

 pencils of rays from the different points of the objects re- 

 presented in the picture do not arrange themselves in a 

 plane, but in a concave surface ; the central rays (passing 

 in or near the axis of the lens) being brought to a focus 

 at a greater distance from the plane of the lens than 

 those Avhich pass through it more obliquely, and form the 

 margins of tlie picture. The amount of this defect varies 

 according to the quality of the lens, and though some- 

 times reduced so far as to be almost imperceptible, 3'et 

 this result can only be attained either by the use of large 

 and expensive lenses, or by unduly curtailing the size of 

 the picture. 



As therefore the foci of rays from all points (which to- 

 gether form the picture) cannot be made to fall strictly 

 on a. plane surface, the object of the following contrivance 

 is so to adapt the receiving surface that all these foci maj' 

 fall upon it, and thus produce approximatelj', if not abso- 

 lutely, a picture of equal distinctness throughout. 



The degree of concavity will vary of course with the 

 forms and different combinations of lenses, and must be 

 determined in each case by experiment. What the ma- 

 thematical nature of the surface may be I am not aware, 

 but practically it may, without risk of appreciable error, 

 be taken as a portion of a sphere. 



Now the substitution of this concave for the plane 

 surface is exceedinglj' simple. A wood frame is made to 

 slide into the groove of the camera in the ordinary way, 

 with a projection all round to receive the edge of the plate 

 which is to support the sensitive paper. This plate con- 

 sists of a sheet of gutta percha, the full size of the interior 

 of the frame, bent to the proper curve, and sufScienth- 



No. 322.] 



thick to retain its shape. The segmental spaces between 

 the margins of the plate and the wood frame are filled in 

 solid, and a rebate cut all round to receive the projection 

 in the frame. A bar or strap is fixed behind to keep the 

 plate firmly in its place ; or the gutta percha plate may, 

 if desired, be permanently attached to the wood frame, 

 A shutter slides in front in the usual way. 



The degree of curvature to be given to the plate is 

 easily obtained by first ascertaining the diflTerence of focal 

 length between the centre and corners of the picture on 

 the ordinary flat focusing glass, thus ; — first get a per- 

 fect focus in the centre, and mark the projection of the 

 lens from the front of the camera on the brass slide ; then 

 slide the lens in until the focus is perfect at the corners, 

 and mark this also ; the distance between the two 'narks 

 on the brass slide will be the difference of focal length 

 required. Then on a piece of cardboard draw a straight 

 line equal in length to the diagonal of the focusing glass, 

 and opposite the middle of it make a mark at a distance 

 equal to the difference of focal lengths as above deter- 

 mined. Through this point and the two extremities of 

 the straight line draw a segment of a circle, and cut the 

 cardboard through neatly with a knife along the line of 

 this segment. Next, procure a convex surface of suffi- 

 cient size, of either glass or polished metal, whose curva- 

 ture coincides with that of the concave portion of the 

 cardboard; and having softened the gutta percha by 

 dipping it in hot water, apply it quickly to the convex 

 surface, and keep it in close contact till quite cold and 

 hard. It is then cut to the exact size, and the segmental 

 spaces under the margin filled in with the same material, 

 so as to give an even bearing all round on the wood frame. 

 Care must be taken that the position of the focusing glass 

 in the camera coincides with that of the centre of the 

 concave sheet. 



When the paper is ready for the camera, it is laid in a 

 moist state on the concave surface of gutta percha, which 

 should be also previously moistened. If neatly laid on, 

 commencing the contact either in the middle or at one 

 Bide, all air-bubbles will be avoided, and the paper will 

 adhere closely and assume the required form. 



The further manipulation need not be described, as it 

 con-esponds in all respects with that usually pursued in 

 the paper processes. 



To render this curved surface available for collo- 

 dion pictures, it is necessary to make considerable modi- 

 fications on that process as hitherto practised. It is 

 evident at once that glass is out of the question as the 

 medium for carrying the sensitive film, although unex- 

 ceptionable in all respects but that of flatness. 



We have then to seek for a material having a perfectly 

 smooth and polished surfiice, tolerably transparent, flex- 

 ible, and insoluble in either alcohol, ether, or water. 

 These conditions may be all fulfilled by the use of that 

 extraordinary material gutta percha. The method is as 

 follows: — Dissolve fifty grains of gutta percha in two 

 ounces of pure chloroform, which is better done if the 

 liquid is slightly warmed: let it stand for two or three 

 days, when all the coarse brown matter will have risen to 

 the top ; this may then be separated, and the solution 

 strained through clean linen. 



Then, on a sheet of perfectly clean glass, placed level, 

 of the size of either one or more pictures, pour a sufficient 

 quantity of this solution to cover it, and protect it from 

 dust. When the chloroform is evaporated, a thin, trans- 

 parent film of gutta percha remains, which may be easily 

 separated from the glass. The upper surface is somewhat 

 dull, but the under surface, having been formed in con- 

 tact with the glass, has all the smoothness and polish 

 possessed by the latter. The film is now taken up by one 

 edge (by means of a pair of broad tweezers, to prevent it 



