RICHARDS AND ARCHIBALD. — GUOWING CRYSTALS, ' 347 



tionary, and the gelatine film was moved, as it is in the common film- 

 cameras. The 2^ inch Eastman cartridge film was found to answer the 

 purpose. At first the turning was effected by an automatic electro-mag- 

 netic arrangement which received its current from a make-and-break 

 contact attached to the shutter. Since a current of ten amperes was 

 needed to secure a sufficiently forcible and speedy action, the operation 

 of this device was somewhat troublesome, and when the exposures were 

 not much more frequent than one a second the film was rolled by hand. 

 A suitable signal attached to the shutter axle, which was still turned by 

 the Henrici motor, gave the necessary indication of the proper moment 

 for renewing the sensitive surfiice. With this apparatus it was of course 

 possible to obtain photographs of isometric crystals, which could not be 

 examined with the preceding arrangement. 



At first a power of 100 diameters was employed, and very satisfactory 

 pictures of the growth of crystals of potassic iodide were obtained. One 

 of these negatives is reproduced here as an example. (Plate II. P^ig. 9.) 

 They showed nothing new, however; hence a much higher power of 

 580 diameters was applied by combining a "2 inch" eyepiece with an 

 "^inch" objective. With this contrivance the light was of course far 

 less intense, and the definition less sharp. Even with the brightest sun- 

 light, concentrated by mirrors and an Abbe condenser, the exposure could 

 not profitably be made less than ^ second. These plates have been 

 enlarged by usual processes to over seven times their original size, so 

 that a total enlargement of over four thousand diameters has been at- 

 tained. Since these larger images are not much more clear than the 

 smaller ones, while thpy occupy much more space, the plates herewith 

 given are all from the original negatives. 



A number of good impressions of crystallizing potassic iodide were taken 

 under these circumstances, but many other rolls were sacrificed. The 

 chief difficulty, as before, was to secure the right moment ; and this diffi- 

 culty was of course much augmented by the limited expanse of the field. 

 Prints from a few of the successful negatives are given below. In order 

 to give a clear impression in printer's ink, these were much intensified by 

 successive photographic printing and intensification ; but of course no at- 

 tempt was made to remove the imperfections of the successive plates, for 

 wiiich allowance may easily be made. (Plate III. Figs. 10-15.) 



The study of these photographs reveals several interesting points. In 

 the first place, it is noticeable that no image is wholly without evidence 

 of crystalline structure. The most doubtful cases are those in Figures 9 

 and 11 ; but the elongated shape of these doulilful images seems to 



