♦ KNOWLEDGE ♦ 



[June 9, 1882. 



close, but considerably unequal pair. < Libra' is a very 

 ■wide and unequal pair, but worth looking at for its pi-ettily 

 contrasted colours. Just to tlie north-west of 5 Serpentis, 

 of whicli we have previously spoken, will l>e found that 

 fine, compressed cluster of very small stars, No. 5 of Messier's 

 Catalogue. It is scarcely resolvable in a 3-in. achromatic, 

 and merely appeai-s like a nebula, brightening conspicuously 

 towards the centre. 



We now arrive at that somewhat unintelligible constel- 

 lation, Hercules, who appears on the maps and globes head 

 downwards between the constellations of the Northern 

 Crown and the Lyre, As our present object, however, is 

 less to endeavour to reconcile the configuration of the stars 

 composing this constellation with the counterfeit present- 

 ment of an invert^'d hero, than to select from them curious 

 and l>eautiful objects, suitable to the instrument we are 

 employing, the map we use will supply all the aid 

 necessary for tliis purpose. We say all the aid ; but, in 

 truth, the map which should give the position of a quarter 

 of the interesting objects with which this constellation 

 teems, would have to be a very elaborate and crowded one 

 indeed, ^^'e must then, perforce, confine ourselves to a 

 few of the most easily identifiable. Beginning upon the 

 confines of Corona Borealis, about 1 \° a little to the east of 

 south of y Corona?, we shall find 23 Herculis. This is a 

 wide pair, but we insert it here for the marked colour 

 of the smaller star, which will be seen below, and 

 just to the right, of its primary. i Herculis, a 

 remarkable binary star, is quite beyond the power 

 of our telescope — in fact, appears single with the 

 means at our disposal If, though, wo fish along a line 

 connecting »; and ; Herculis about one-third of the way 

 »), we shall light upon an object which will amply repay us 

 for any disappointment we may experience in connection 

 with this. The object to which we refer is No. 13 of 

 ilessier's Catalogue, and consists of a most glorious globular 

 cluster of stars. How far we shall succeed in detecting 

 its stellar character will depend upon the excellence of our 

 instrument and the acuteness and training of our \'ision. 

 We liave tried to indicate its character in Fig. 38. One- 

 and-a-half degrees N. ))y E. of j) Herculis will be found 

 another cluster (Messier 92), which the average eye and 

 instrument will only show as a bright nebula. We may 

 further note here that there are two planetary ncbuhe in 

 this constellation ; but that only one of these is at all 

 within the reach of a three-inch telescope ; and neither can 

 l>e found with certainty, save in an equatorially-mounted 

 telescope. 



PHOTOGRAPHY FOR AMATEURS. 



By a. BKOxnERS, F.R.A.S. 



IF kept standing in the dark slide for any length of time, 

 the sf;nsitive collodion film will dry, and the time it 

 will remain in a condition fit for use will depend on the 

 tempfjrature of the air. Another disa<lvantage is that the 

 silver solution requirf^l to form the picture while under 

 development drains away, and if the weather be very hot, 

 and the comers of the dark frame are not clean, markings, 

 known as " oyster-shell," are likely to form, and often spoil 

 the negative. Two or three folds of red blotting paper wettf^d 

 and placed at the back of the plate will retard evaporation 

 and keep the film sensitive for a considerable time, and if 

 the plate have lost much of the silver, it may be restored 

 by redipping in the silver solution. But aft<;r all, the results 

 on plates so preserved are seldom satisfactory. 



Various methods have been adopted for keeping the 

 plates in a moist state, such as coating with solution of 

 sugar, honey, glycerine, or other suitable vehicle that will 

 retard the drying of the film. It is obvious that if the 

 sensitiveness of the film could bo retained in a dri/ state, 

 we should ha^•e a convenient substitute for the wet 

 process. The labour involved in working with wet 

 collodion away from home conveniences, the necessity 

 for some kind of tent, the possibility of breakages, 

 and occasionally the discovery that some important 

 article has been left at home — all these disadvantages 

 set many minds working to discover a practicable 

 drj/ process. To enumerate or describe the processes at 

 various times introduced would be tedious. Most of them, 

 in the hands of their discoverers, gave excellent results, 

 and some of them came into general use. The collodio- 

 albumen process may be named as one of the best, and has 

 been used very extensively. It has the disadvantage, 

 however, of being very slow, but the results, in the 

 hands of careful workers, have been quite equal to wet 

 collodion. It will be sutlicient to say here that the plates 

 are prepared, first, with bromo-iodized collodion in the 

 usual way. On the removal of the plate from the bath, 

 it is washed and then put into a weak solution of iodide 

 of potassium and again washed. The plate is then coated 

 with albumen, containing iodide and bromide of potassium 

 in various proportions, and also liquor ammonia. It is 

 then dried by a fire, and is ready to be sensitised, or rather 

 to bo re-sensitised, which is eflccted liy immersion in a bath 

 of nitrate of silver in the ordinary way, and it is again 

 washed. The plate is then allowed to dry without the 

 application of heat. The plates will keep in good condition 

 for many months, and the latent image may be developed 

 at pleasure, and after the lapse of many weeks or months. 

 The development is effected by using pyrogallic acid, 

 citric acid, and nitrate of silver in certain proportions ; and 

 hyposulphite of soda is used for fixing the pictures. 



I liave described this process in general terms first, 

 because it gives most excellent results, and should not be 

 passed over ; and, next, because the amateur, if he desire 

 to make use of it, will seek particulars in treatises devoted 

 to the subject. 



Another method by which the silver bath may be 

 dispensed with is by adding nitrate of silver to the collo- 

 dion, forming an emulsion which may bo poured on to the 

 plate in the ordinary way, and when dry is ready for use. 

 The development is eliectcd by the alkaline method. 

 1 ,The practice of out-door photography may be said to 

 have been entirely changed during the last three or four 

 years by the introduction of the ;/elatine dry plates, and 

 not only for outdoor work, for these plates have now 

 almost superseded collodion and the silver bath in the 

 studio work of the professional j)liotographer — at least for 

 portraiture. The extraordinary rapidity of the gelatine 

 film has effected this result. As compared with collodion, 

 the time of exposure necessary to oljtain a picture; has been 

 reduced to the fraction of a second ; so rapid, indeed, is the 

 action of light upon the plate, that, for out-door work, the 

 effect is as instantaneous as the exposure can be made by 

 mechanical means, and for studio work the change may 

 be said to be ten or twenty times hjss than by the old 

 method. 



In a future p9.per we may return to the subject, and 

 describe the method of preparing the gelatine plates. To 

 do so now would be unprofitable to the tyro, owing to the 

 delicacy of some of the operations. The plates can be 

 purchased ready for use, of various degrees of sensitiveness, 

 from five to twenty times that of wet collodion, and, if 

 desired, this degree of sensitiveness may be exceeded ; 



