346 Dr. J. H. J. Poole on the Minimum 



was re-polished by Hilger Ltd., as it was found that there 

 was rather an undue amount o£ irregular reflexion taking- 

 place at the surface of the mirror. To avoid, as far as 

 possible, any stray light from the mounting of the mirror 

 or from its back reaching the eye, they were both carefully 

 blackened by means of a small turpentine flame, which gave 

 a very good deposit of lamp-black. 



The chief difficulty experienced was with the optical 

 arrangements. It is plain that it is impossible to realize the 

 ideal of a perfectly parallel beam owing to the finite size of 

 the light-radiant. The best we can do is to use as small a 

 radiant as possible, and also as long a focus lens as we can 

 without reducing the illumination too much. In the preli- 

 minary experiments a slit was used as the radiant, the light 

 from a projection type of electric glow lamp being concen- 

 trated on it with a condensing lens. Between this slit and 

 the mirror a long focus lens was placed, and the distance of 

 the lens and the slit was so adjusted that the image of the 

 latter was formed on the second slit behind which the eye 

 was placed. The lens is naturally placed as close to the 

 revolving mirror as possible, as the image formed will be 

 magnified in the ratio of its distances from the two slits, 

 while the speed of the image will be proportional to the 

 distance of the mirror from what we might perhaps call the 

 eye-slit ; thus the distance between the lens and the mirror 

 only tends to enlarge the image without increasing the speed, 

 and should hence be kept as small as is convenient. In the 

 first trials this lens was of about 2 metres focal length, which 

 was hardly sufficient. These experiments were conducted 

 in an ordinary room, and it was found impossible to keep it 

 sufficiently dark for the eye of the observer to become truly 

 dark-adapted. The distance between the mirror and the 

 eye-slit could only be made about 15 metres, a distance 

 which it was thought could be increased with advantage. 



On both these accounts it was decided to remove the 

 apparatus to the cellars of this building. These cellars 

 were particularly suitable for the purpose, as a distance of 

 30 metres could easily be obtained between the mirror and 

 the eye-slit, and no trouble was experienced in keeping 

 them nearly perfectly dark. Some modifications were also 

 introduced into the arrangements as a result of our previous 

 trials. Thus we had already found that even at the maximum 

 speed of the centrifuge the light was plainly visible in the 

 mirror. This showed that a flash of the order of about 

 10 -6 sec. or rather less was visible if repeated sufficiently 

 often. To be able to deal with one flash of this duration, 



