4 J. AITKEN ON THE NUMBER OF DUST PARTICLES IN THE ATMOSPHERE. 



of thin and carefully selected glass was used instead of the mirror. The glass had fine 

 lines etched on its surface. This arrangement gave very fair results, though not so satis- 

 factory in some respects as the mirror, if only the double image of the latter could be got 

 rid of. Attempts were therefore now directed to see if the mirror arrangement could not 

 be improved. To get rid of the double reflection, a silver mirror seemed to be the best 

 thing, if it could be got sufficiently perfect on its surface. My first attempts in this 

 direction wen' made with silver deposited on highly polished glass plates, the silvered side 

 of the glass being used. These mirrors gave highly satisfactory results, but were much 

 too delicate, being easily destroyed by water. With some difficulty, I however succeeded 

 in getting plates of solid silver with a surface sufficiently perfect for the purpose. 



Silver mirrors, with fine lines ruled on them at 1 millimetre apart, are now the only ones 

 used, and are found to work very well, though they require considerable attention to keep 

 them in working order. Before beginning work, the mirror is always polished, occasion- 

 ally on a buff wheel with rouge, and finished off on a wool wheel. In polishing, care has 

 to be taken that the plate is always rubbed in straight lines in one direction and along 

 one set, and that always along the same set of ruled lines and across the other set, no 

 cross or circular rubbings being made. The reason for this is, that, when the rubbing 

 marks are all in one direction, it is possible so to arrange the direction of these rubbings 

 with regard to the position of the light that the surface of the mirror will look black, and 

 the drops shine out brightly on its surface. 



Over the receiver is placed a cover, blackened inside to exclude all light except what 

 comes from the gas flame, and an opening is made in the top of it through which the 

 stage is viewed by means of a lens. The mirror requires care in its adjustment with 

 regard to the position of the light to enable the drops to be easily seen. It should 

 be turned so that a diagonal to the little squares on its surface will point to the light. 

 When the lighting is properly adjusted, the mirror appears black on looking through 

 the lens, while the lines are distinctly but not brilliantly illuminated, and when the 

 drops fall on the mirror they show as bright specks on the black surface. 



An interesting illustration of the spheroidal condition is often seen in these experiments. 

 When the stage is very slightly warmer than the drops, they do not adhere to the plate, 

 but roll on its surface, moving towards the lowest part. They glide over the little 

 squares till they meet a boundary line into which they fall, and in which they roll down 

 till quite evaporated. So little resistance do they meet with in their movement, that it 

 is almost impossible to make the stage so perfectly horizontal that they will not roll one 

 way or another. This rolling of the drops interferes much with the counting, but, as it 

 can be easily avoided, counting should never be done while the stage is in this condition; 

 because, in addition to the difficulty of counting, some of the drops evaporate before they 

 can be counted. Glass plates seem to be more liable to this difficulty than the silver 

 ones, probably owing to glass being a better absorber than silver of the radiant heat of the 

 illuminating flame. 



