The Correction of Telescopic Objectives. 



405 



recoil phenomena due to /3-rays must be considered as 

 hopeless. 



The results given in the present paper also throw some 

 light on the question of the charge of the recoil atoms. 

 Apart from the fact that the recoil atoms from the emanations 

 are usually believed to be partly negatively charged in order 

 to explain the origin of the anode activity, the atoms of RaB 

 given up by RaA were also found to carry partly a negative 

 charge. To account for the activity of a positively charged 

 collecting disk placed over a plate coated with RaA, 

 Wertenstein* and others have assumed that from 2 to 

 5 per cent, of the recoil atoms of RaB carry a negative 

 charge. In all these cases the activity of the collecting disks 

 (probably consisting of RaA) was not analysed. In some of 

 the experiments described in this paper, when the disk has 

 collected about l0> 1 000 of the total amount of RaA on the 

 plate, no traces of RaB could be found on the disk. Since 

 the presence of RaB in the proportion of ^o °f the amount of 

 active matter could easily be detected on the disk, it follows 

 that the proportion of recoil atoms of RaB carrying a 

 negative charge is certainly less than 1 to 100,000. 



My best thanks are due to Prof. Sir Ernest Rutherford 

 for his kind interest in this work and for the supply of large 

 quantities of radium emanation. 



XLIV. The Correction of Telescopic Objectives. By T. 

 Smith, B.A., Optical Department, National Physical 

 Laboratory f . 



IN the Philosophical Magazine for June Mr. A. O. Allen 

 has pointed out the possibility of expressing in a small 

 compass all the information contained in the N. P. L. tables 

 of constructional data for small objectives, and much more 

 besides, by means of a few formulae and other methods. He 

 gives expressions for this purpose and works out a number of 

 numerical illustrations. The formulae as he presents them 

 are open to criticism, and the same may be said of several 

 statements made in the course of the paper. I propose in 

 this note to deal briefly with a few of the more important of 

 these. 



* Wertenstein, These, Paris 1912. 

 t Communicated by the Author. 



