TRANSACTIONS OF THE SECTIONS. 19 



" distance of the picture," the rays thus entering the retina from all parts at nearly 

 the same angle, instead of those from the centre being full and direct, and those 

 from the extremities weakened by obliquity, as would be the case if the picture were 

 the size of life. 



The remedy proposed was the employment of a second artificial negative, similar to 

 the tint stone in double lithography. In preparing this, a sheet of transparent tracing 

 paper is laid upon the original photograph, and all those portions which are to form 

 the high lights are stopped out with opake colour, the clouds being formed with 

 washes by a camel-hair brush, and the fine tracery of architecture, &c. with a reed, 

 pen or crowquill. Whilst the positive impression is still sensitive, this tint pE^per is 

 to be accurately fitted on, either by the eye, or points in the frame, and exposed 

 again to the light, until flat tones of the requisite depth are produced. The sky may 

 be graduated by moving a shade over the surface, allowing the horizon to be least 

 exposed, the effect of which is to produce the utmost delicacy in the force of the 

 clouds as they recede in the perspective, however rudely drawn. A still more per- 

 fect method is to commence with a good negative photograph of' natural clouds, 

 proceeding as before with the details of the picture. 



The first object should be to throw a flat tint over all those portions which, from 

 being blue, have printed too light, as the sky, slate roofs, and all polished surfaces, 

 as water, leaves of evergreens, &c., which reflect the blue of the sky. This alone 

 will often produce a pleasing picture from a very unsatisfactory negative, light objects 

 relieved by a dark sky, and the deep tones of water, especially in sea pieces, being 

 amongst the most effective objects in nature, but which are quite lost in ordinary 

 photography. The effect must be left to the skill of the artist, shadows of clouds, with 

 the toning down of obtrusive or offensive features being amongst the most obvious 

 means employed to improve the composition, without interfering with the truth of 

 the original outlines. The most powerful effects of moonlight, sunrise, or sunset 

 may thus be produced, with reflexions in still water, or the gleams and ripple of a 

 breeze ; the foam and sharp lines of a cataract may, by stopping out high lights on 

 the original negative, be also substituted for the dull mass which ordinarily repre- 

 sents falling water. 



The difiicultj' of reversing the lights of the clouds by using dark colour, may, if 

 preferred, be avoided by substituting Chinese white tinged with yellow, the trans- 

 parent paper being laid upon the dark sky of the negative. 



A tint paper thus produced may be used for any number of impressions, and, if 

 the details of the picture are satisfactory, skies alone may be adapted to many 

 different negatives, especially if drawn of more than the requisite extent, so as to 

 apply such portion as is suitable to the composition of each picture. 



Many other suggestions were offered, such as inverting the negative to produce 

 reflexions, when taken from the level of the water, the introduction of foregrounds, 

 cattle, &c., by using both the object and the matrix from which it was cut, so that 

 the lines should exactly coincide. Methods were also shown by which the printing 

 of parts of a photograph may be retarded so as to bring up the more opake portions. 

 The most effectual of these was to attach a sheet of transparent tracing paper over 

 the back of the negative and to stump over the weaker parts, so as when seen by 

 transmitted light the whole should be in due gradation. The dispersion of ray 

 through the thickness of the glass is found sufiicient to prevent any trace of thia 

 artificial shading. With paper negatives the same result may also be produced by 

 partial waxing. 



On the Construction and Use of an Instrument for determining the Value of 

 Intermittent or Alternating Electric Currents for purposes of Practical 

 Telegraphy. By Wildman Whitehouse. 



In the prosecution of some electrical studies, requiring an estimate of the vdues 

 of different magneto-electric currents, Mr. Whitehouse found that the ordinary gal- 

 vanometer was totally inadequate to indicate the required results. 



However suitable that instrument might be for a continuous or voltaic current, 

 and within a very limited range, yet the problem before him involved the numerical 

 estimate of currents of the widest range and of the shortest duration. 



2* 



