Xo. rV.] APPENDIX. 161 



are apt to absorb the solutions unequally. Thrae papers are fittest when 

 the oonunon salt and nitrate of sUver are used. 



The highly-glazed papers, or writing papers, require no particular 

 observation, for if either chlorine, chloride of lime, or chloride of soda be 

 used, the colour will be found uniform; and the finer and more highly 

 glazed the paper is, the better will it suit the intended purpose. These 

 will be found advantageous, not only from possessing a smooth and 

 uniform colour, but also from a smaller quantity of the solution of 

 nitrate of silver being used in their preparation, as it is applied only on 

 the surface, and does not penetrate any distance into the texture. For 

 this latter property, paper such as the satin post may be prepared on 

 both surfaces, should that be deemed advisable. 



The modes of applying the chemical substances to the paper have 

 been already noticed, and the sponge was mentioned as being the agent 

 employed. 



The extent to which the paper should be moistened is, that such a 

 quantity of solution should be used, that it may, as artists term it, 

 " bear out " in every part of the surface ; that is, that a slight layer of 

 moistore should appear at every point after the usual absorption has 

 taken place, and that all superfluous moisture is to be carefully removed 

 by a pressed sponge. 



After the paper has been prepared, it will be hardly necessary to state 

 that it must be kept caref ally from the action of the light. 



The mode of making the drawings has been sufficiently detailed in 

 various publications. When prints are to be copied, the printed side 

 must be pressed by a piece of flat glass close to the prepared paper, and 

 exposed to the light of the sun. When drawings of feathers or other 

 irregular bodies are desired, a piece of the photogenic paper is to be laid 

 upon any yielding substance, as folded Unen, flannel, or, what is perhaps 

 better, a layer of sand or bran ; the object is then to be covered with a 

 square of flat glass, and, if necessary, pressed down by weights, and is to 

 be finally exposed to the light of the sun. 



The paper vrill be found to be most rapidly acted upon by the direct 

 rays of the sun, but this is by no means indispensable, as a clear sky is 

 very efiectnal, and even on a very cloudy day a delineation is produced, 

 only it requires a longer time. The circumstances which appear most 

 to retard the photogenic properties of the solar beam, are those dense 

 collections of smoke which hover over the metropolis when the wind has 

 not sufficient power to disperse the deleterious particles of which they are 

 composed. 



Most of the modes of preparing the paper which have been described, 

 are applicable to the camera obscura with a short focus; and those 

 prepared with the chloride of soda, chloride of lime, and bromide of 

 potassium, do extremely well. Its use in this department will for ever be 

 limited, for a portion of an object only can be represented accurately, as, 

 for every distance, the camera requires a different adjustment of its focus ; 

 so that to take a landscape a hundred different foci would scarce suffice. 

 For this reason, it certainly appears that the results of M. Dagnerre's 

 .experiments must be exaggerated. 



The fiTTiTig of the drawings after they have been made is completely 

 a chemical action, and requires as much care as the preparation of the 



