76 Intelligence and Miscellaneous Articles. 



may be rubbed strongly with cotton and water without any injury 

 to it ; but, on the contrary, with much improvement, as this removes 

 any particles of dust or other impurity, and gives the whole picture 

 a fresh degree of vivacity and lustre. A Daguerreotype picture would 

 be destroyed by such rough usage before it was completely fixed and 

 finished. 



In examining one of the Amphitype pictures, the first thing that 

 strikes the observer is, the much greater visibility of the positive 

 image than of the negative one ; which is at least in the proportion 

 of ten to one, since it is not rare to obtain plates which are almost 

 invisible by transmitted light, and which yet present a brilliant pic- 

 ture full of details when seen by reflected light. 



The object of giving to the plates a second coating of albumen, as 

 prescribed in No. 4, is chiefly in order to obtain this well-developed 

 positive image ; for it is a most extraordinary fact, that a small 

 change in the relative proportions of the chemical substances em- 

 ployed enables us at pleasure to cause the final image to be either 

 entirely negative or almost entirely positive. In performing the ex- 

 periment of the rotating wheel the latter process must be adopted, 

 since the transmitted or negative image is not strong enough to be vi- 

 sible unless the electric flash producing it bean exceedingly bright one. 



I now proceed to mention a peculiarity of these images which ap- 

 pears to me to justify still further the name of Amphitype, or, as it 

 may be rendered in other words, " ambiguous image." Until lately 

 I had imagined that the division of photographic images into positive 

 and negative was a complete and rigorous one, and that all images 

 must be of either the one or the other kind. But a third kind of 

 image of a new and unexpected nature is observed upon the Amphi- 

 type plates. In order to render this intelhgible, I will first recall 

 the general fact that the image seen by transmitted light is negative 

 and that by reflected light positive. Yet, nevertheless, if we vary 

 the inclination of the plate, holding it in various lights, we shall not 

 fail speedily to discover a position in which the image is positive 

 although seen by transmitted light. This is already a fact greatly 

 requiring explanation. But the most singular part of the matter is, 

 that in this new image (which I call the transmitted positive), the 

 brightest objects (viz. those that really are brightest, and which ap- 

 pear so in the reflected positive) are entirely wanting. In the places 

 where these ought to have been seen, the picture appears pierced 

 with holes, through which are seen the objects which are behind. 

 Now, if this singularity occurred in all the positions in which the 

 plate gives a positive image, I should be satisfied with the explana- 

 tion that the too great brightness of the objects had destroyed the 

 photographic effect which they had themselves at first produced. 

 But since this effect takes place in the transmitted positive but not in 

 the reflected positive, I am at a loss to suggest the reason of it, 

 and can only say that this part of optical science, dependent upon 

 the molecular constitution of bodies, is in great need of a most careful 

 experimental investigation. 



The delicate experiment of the revolving wheel requires for its 

 success that the iodide of iron employed should be in a peculiar or 



