1882.] 



MICROSCOPICAL JOURISTAL. 



91 



such as I have here, I shall en- 

 deavor to perform the operation this 

 evening so that you may all watch the 

 development of the image. The de- 

 veloping solution consists essentially 

 of some reducing agent — sulphate of 

 iron, for example, or pyrogallic acid. 

 These, when properly applied to the 

 surface of the exposed plate, cause a 

 reduction of the silver over those 

 parts which have been acted upon 

 by the light, and all those parts be- 

 come dark and more or less opaque. 

 It is in those parts of the negative, 

 which correspond to the light parts of 

 the object, that the details of the 

 picture are to be looked for as the 

 development proceeds. After the 

 picture is fully developed it is washed, 

 and fixed in sodic hyposulphite. 

 This removes all the unchanged sil- 

 ver bromide, and clears the plate. 

 The plate is then set aside to dry, 

 and is afterwards varnished, when it 

 is ready to be printed from. 



For photographing from the micro- 

 scope by lamplight, it need hardly be 

 said that the plates should be very 

 sensitive to light. It might be in- 

 ferred, therefore, that the most sensi- 

 tive plates are the best for this pur- 

 pose. But it is doubtful if this is so. 

 I am not able to speak from experi- 

 ence in this matter, never having used 

 the most rapid plates, but I should 

 expect to find more detail in a nega- 

 tive from one of the less rapid plates. 

 I have made use of only one kind of 

 plate — the " Keystone Rapid " plate 

 " B," and the results have been qui^e 

 satisfactory. The time of exposure 

 varies with the structure to be photo- 

 graphed. The object I have chosen 

 for this evening is one which I have 

 not previously tried, an insect mount- 

 ed in Canada balsam, but the 

 image leads me to think a compara- 

 tively short exposure will suffice. 

 To photograph an object which is 

 yellowish or red, although the form 

 of the object would be quickly im- 

 pressed upon the plate, to obtain the 

 details — the delicate shading of the 

 different parts, which is necessary to 



give character to the photograph — 

 would require a much longer time. 

 The reason for this is that the red and 

 yellow rays act very slowly upon the 

 silver-salt. Therefore no definite in- 

 structions can be given. However, 

 yellow rays, as Prof. Kain has ob- 

 served, act upon the dry-plates with 

 comparatively greater activity than 

 they do upon collodion wet-plates. 

 In Prof. Kain's article upon this sub- 

 ject, published last month, the rela- 

 tive exposures for several different 

 objectives is given. 



The highest power I have yet used 

 with this apparatus is a so-called }^- 

 inch and the lowest ocular, which gave 

 a magnification of about 300 diame- 

 ters. The combination that will be 

 used this evening is a Gundlach i -inch 

 and the same ocular. The magnifica- 

 tion thus obtained is 85 diameters. 

 I shall expose the plate about one 

 and a half minutes. 



I see no reason why a power 

 as high as a J^-inch objective cannot 

 be employed with satisfactory results 

 with the same source of light. How- 

 ever, the electric light will doubtless 

 be available for this purpose before 

 long, and with that there will be no 

 difficulty in using very high powers, 

 as Dr. Van Heurck has already done. 

 But if lamplight proves inadequate 

 for the higher powers, sunlight can 

 be employed with the same apparatus. 

 Moreover, with the dry-plates sunlight 

 can be used without a heliostat, for 

 the exposure required is so very brief 

 that the sun will not leave the object 

 before the exposure is made, if one 

 works expeditiously. While writing 

 this I received a letter from Mr. John 

 Carbutt, of Philadelphia, who has de- 

 vised a lantern, the patent for which 

 was issued last month, which he 

 thinks will be of great assistance in mi- 

 croscopical photography I do not 

 know what special advantages are 

 claimed for this lantern, but probably 

 it gives a very intense light. 



Those who wish to undertake the 

 work of photographing from the mi- 

 croscope will, I am sure, find it very 



