112 A CONTINUOUS RECORD OF ATMOSPHERIC NUCLEATION. 



are not as immediately available for quantitative discussion as was hoped. 

 The number of nuclei per cubic centimeter and the diameters of the fog par- 

 ticles are respectively below and above the computed nucleation (Chap. VI), 

 and the globules photographed are rarely of the same size for a given corona. 



Very curious results, apparently capillary in character, were obtained, 

 showing permanent pitting effects of the subsidence of fog on a film of viscous 

 oil and persistent motion of globules in liquid oil. 



9. Apparatus and method. The apparatus needed is a modification of that 

 described in 2 of this chapter for the microrr.etry of fog particles, with the addi- 

 tion of a camera above the microscope. The usual form of camera attached to 

 a substantial eccentric axis so as easily to be rotated into place or removed 

 therefrom for inspection is satisfactory. The revoluble condensation chamber 

 must be clamped in place. The camera may be focussed by the stage focussing 

 screw of the microscope, so that a lens is apparently not necessary in the camera ; 

 but it is essential that the magnified fog particles be seen very clearly with the 

 eye at the microscope, before photographing them, and this procedure is there- 

 fore not quick enough. It was thus found necessary to add a good lens to the 

 camera adjusted for parallel rays and to adapt the eye for the same infinite 

 focus by concave glasses. In this case, when the particles had been caught and 

 put in place under the objective by the eccentric stage device described above 

 ( 2), the camera could be at once swung into position for photography. The 

 endeavor to adapt a small kodak for the work was not very successful. 



Magnification may be secured either at the objective, or at the ocular. 

 Some space between the objective and the plate is desirable for safe manipula- 

 tion, and therefore a half -inch objective will serve the present purposes better 

 than a quarter-inch lens. The illumination must be axial to avoid astigmatism, 

 but the use of condensers has not as yet been tried, and would probably pro- 

 mote evaporation of the fog particles. In general, reasonably small magnifica- 

 tion, much light, and rapid photography are best conducive to success. In this 

 way the time of exposure was gradually reduced from 20 seconds to 2 seconds 

 or less. The positive ocular seen in the plates was used merely in the absence 

 of a suitable negative ocular, inasmuch as the former was provided with a filar 

 ocular micrometer. But a negative ocular containing a plate ruled in square 

 millimeters would be far preferable. 



The plate in these experiments was covered with an even coating of Damar 

 varnish, neither too moist nor too dry. A clean microscope cover-plate of glass 

 is dipped in the varnish, and the excess removed by placing it on edge. The 

 film, which must be smooth, clear, and even, will be ready for use in a few hours. 

 If too dry it should be soaked in turpentine, otherwise the particles adhere 

 broadly to the plate and rapidly evaporate. If the plate is too moist, the 

 particles float and cannot be photographed. No precise rules can be given. 

 Naturally, the conditions surrounding the plate should be as nearly as possible 

 isothermal, which implies a capacious air chamber for condensation. 



10. Incidental phenomena. Pitting. A curious phenomenon sometimes 



