376 



Mr. Stokes thought this paper was a great acquisition to the 

 Society, and he personally felt very grateful to Mr. Cheshire for 

 bringing it before them. The formulae, he believed, were new, 

 and were certainly likely to be very useful. He had seen Professor 

 Thompson's focometer, which was a very elaborate and also a very 

 expensive appliance, the cost running nearly to three figures, and 

 therefore quite beyond the reach of most microscopists. Simple 

 methods like those described in this paper were therefore a great 

 acquisition, especially as they were shown to be so reliable as to 

 their accuracy. It was a very important thing to determine 

 numerical aperture, because it gave them the limit of the resolving 

 power of an objective. He thanked Mr. Cheshire for this paper, 

 which he was sure would be read with very great pleasure and 

 profit when it appeared in the Journal. 



Mr. Rheinberg thought the methods described would be very 

 useful to those who desired to obtain for themselves results which 

 they could rely upon as being accurate, because at the present 

 day they saw so many things and heard so many methods proposed 

 that they sometimes did not know where they were. A great 

 advantage of these methods was that many of the experiments 

 described could be carried out by any one without any apparatus 

 whatever. It was curious to note that the method described of 

 measuring the focal length of a lens occurred to Mr. Nelson 

 also at the same time as to Mr. Cheshire, and was described 

 in the last number of the R.M.S. Journal. He wished to ask 

 Mr. Cheshire as to the particular adjustment necessary to obtain 

 the very curious figures which he had described, and what would 

 jDroduce the oblique form of the straight rod as shown. 



Mr. Cheshire said that in describing his method of apertometry 

 the one thing he would insist upon was that it insured results 

 equal to Zeiss's. It must be remembered, however, that it could 

 not be used with an achromatic condenser, because it was a 

 necessity of the case that the scale must be put at the back-focal 

 plane of the condenser, and in the achromatic form they could 

 not get at this point. As an illustration of the usefulness of 

 being able to easily test objectives, he might say that, seeing a 

 description of a lens in a maker's catalogue which was said to 

 have an N.A. of -55, he tested two of them, and found they were 

 really -48 and -47 only ; but on calling attention to it he was told 

 that this was a misprint — they really were •48. On another 



