462 



SCIENTIFIC NEWS. 



[May i8, if 



thermometer^ shines through that part of the column 

 not occupied by mercury, which, in the case of the dry 

 bulb thermometer, is all that part of the tube above the 

 mercury, and in the case of the wet bulb thermometer a 

 long air-bubble occupying about 12° of the scale. The 

 light passing through the thermometer-tubes is received 

 on sensitized paper placed on a revolving vertical cylinder 

 immediately behind the thermometers, which stand side 

 by side, the light in each case entering at such an angle 

 that the record of the wet bulb thermometer appears 

 precisely below, on the paper, that of the dry bulb, the 

 lower edge of each trace being the properithermomelric 

 indication to be employed. Degree-marks cut on the 

 stems of the thermometers and painted, appear on the 

 photographs, when developed, as white lines, the 

 lo-degree lines being thicker than the others. Thus the 

 photographs carry their own degree scales. The driving 

 clock at each hour interrupts the traces for a short time, 

 giving corresponding hour lines on the registers. The 

 apparatus is enclosed in a metal case, the bulbs of the 

 thermometers projecting below, and being about 4 ft. 

 above the ground. The whole apparatus stands under a 

 louvreboardedshed, under which there is free circulation of 

 air. The apparatus was constructed by Negretti and 

 Zambra. The illustration supplied by Mr. McClellan, 

 of the Magnetical and Meteorological Department, is 

 from a reduced copy of a view taken by Negretti and 

 Zambra. One side of the metal case is shown removed, in 

 order that the general internal arrangement may be seen. 



PHOTOGRAPHIC LENSES. 



The lens is the most important item of the photographer's 

 outfit. To be thoroughly equipped for all kinds of work 

 three types of lenses are essentia], namely, a single, 

 a rectilinear doublet, and a wide-angled one. 



Most writers in the photographic primers recommend 

 the amateur who can only afford one lens to begin with, 

 to purchase a " rectilinear doublet." This, however, is 

 not the best advice. 



For all-round work, save of course purely architectural 

 subjects, nothing can compare to a really first-class single 

 lens. For speed, for flatness of field, for depth of focus, 

 in fact for everything, except rectilinearity, nothing can 

 approach it. 



By " depth of focus," of course, is meant the power in 

 the lens to represent sharply objects near, as well as 

 those at a distance from the lens. By "flatness of 

 field " is meant the power in the lens to give perfect 

 definition even to the margins of the picture. 



Naturally, every beginner wishes to possess a quick- 

 acting lens, but he must bear in mind what he gains in 

 speed he loses in flatness of field and depth of definition. 

 By " width of angle " is meant the " sweep " of picture 

 given without falling off of definition towards the 

 margins. 



The width of angle of a lens entirely depends upon the 

 relation of its size to its focal. length ; that is to say, the 

 shorter the focus the larger the angle, the longer the focus 

 the smaller the angle. 



Very wide-angled lenses are dangerous tools to use, 

 especially for architectural subjects in cramped positions, 

 as frequently they will so distort a view as to render it 

 almost unrecognisable. Its legitimate field is more in 

 landscape photography, to which absolute accuracy may 

 be reasonably sacrificed to artistic effect. 



The best form of view lens is the " single meniscus " 



type, which undoubtedly gives the most brilliant pic- 

 tures. 



It is equally as rapid as the " doublet " worked at the 

 same focal aperture, if not even more so, although many 

 amateurs are under a false impression on this point. As 

 Captain Abney points out in his splendid treatise on 

 photography, " the loss of light from reflection by the 

 surface in the single type of lens is the least possible." 

 Its only drawback is that it gives slight distortion, 

 rendering straight lines near the margins curved or 

 barrel-shaped. It is therefore unsuitable for architectural 

 subjects. This, however, will not be so much noticed 

 if the straight lines of the building do not occupy the ex- 

 treme edges of the picture. 



Other things being equal, the smaller the diameter the 

 better the lens. 



For architectural subjects, interiors, some kinds of in- 

 stantaneous effects, copying, and enlarging, a rectilinear 

 doublet is the best, and one of moderate angle is to be 

 preferred. 



Exposure is the great bugbear of the beginner, and 

 this because many amateurs are in utter ignorance of the 

 focus of their lens, and the proper use and value of their 

 diaphragms. 



If the focus of a lens be not known, its discovery is a 

 simple matter. Sharply focus some object over twenty 

 yards distance. Then measure accurately the distance 

 between the focussing ground-glass, and the single lens, 

 or if the lens be a doublet, from the ground-glass to the 

 diaphragm slit. This will be the focal length of the lens. 

 Next measure very carefully the diameter of each stop 

 aperture. Then, in order to find the focal value of 

 each stop, divide the length of focus by each diameter. 

 For example, say the focal value of the lens is nine 

 inches, and the diameter of the largest stop one inch : 

 then 9 -r I = 9, so that the value of that particular stop 

 will be expressed by F 9. 



It goes without the saying that the smaller the stop 

 used the longer the exposure required. 



In order to ascertain the relative exposure of stops, 

 square the focal numbers. 



For instance, if the correct exposure of a landscape 

 with Fig stop was one second, F12 would require i| 

 seconds, F 16 2i seconds, and F 40 16 seconds. 



It is thus evident that lenses are more or less rapid in 

 action as their working diameters are larger or smaller, 

 or as their equivalent focal lengths are shorter or longer. 

 The size of the picture too is determined by the focal 

 length. The proper distance to place subjects for por- 

 traiture from the lens should not be less than twelve 

 feet or more than thirty. 



In portraiture, too, tilting the camera forward is an 

 advantage rather than otherwise, as it produces a more 

 pleasing view of the face than if the camera was per- 

 fectly level. 



Just a final word or two on the proper use of stops. 

 The most brilliant results are obtained by using the 

 largest possible stop, which will bring all the picture into 

 the requisite focus. By using a large stop, breadth and at- 

 mosphere, as well as brilliancy, are secured in the picture. 

 Always make a rule of focussing the central object of 

 interest in the picture with the largest stop. This done, 

 insert the next largest stop, which will give the'necessary 

 sharpness all over the picture. The distance should not 

 be rendered, however, too sharp, if the picture is to be 

 artistic. 



Always carefully dust a lens before using it. 



