ADJOURNED DISCUSSION IN LONDON 245 



Mr. Beck: Will you explain to us whether a quantitative 

 aneasurement is obtained in the star test. The star test has been in 

 use with the microscope objective ever since the achromatic micro- 

 scope objective was known, in the form of a minute mercury globule 

 reflecting a small source of light which makes practically an^ artificial 

 star. 



Mr. Martin : The work I have been doing is in a very unad- 

 vanced stage, but I hope it will be possible to obtain a rough esti- 

 mate of the variation of the spherical aberration. 



Commander Ainslie: I had the curiosity to test a low power 

 objective on the well-known AVassel method, and it was easy to 

 -obtain (by this particular method of the extinction of the two sides 

 of a zone simultaneously, with a screen), numerical values for the 

 different foci of the different zones. I was only using a low power 

 •objective, an half -inch apochromat, and it would Be difficult vdth 

 liigh powers, unless, perhaps, an auxiliary telescope is used. 



Mr. T. Smith : Mr. Beck said that the Hartridge test would 

 not give coma. May I suggest that it is quite easy to get coma by 

 plotting the spherical aberration for two somewhat different magnifi- 

 cations. From these numerical values, the deduction of the coma 

 is quite easy. 



Professor Eyre, in bringing the discussion to a close, 

 said : 



The time has now come when I must close the 

 meeting. It is very difficult at the end of an evening o 

 this character to sum up with anything like precision or to 

 offer an opinion that has any value on the work that has 

 been presented. There is, however, one outstanding 

 feature, namely, that workers are willing and anxious to 

 state their requirements to the manufacturers, and I think 

 we have evidence that the manufacturers on their side are 

 willing to do all in their power to help meet these needs. 

 We cannot expect perfection at once. As Mr. Watson 

 Baker has said, it has taken quite a year to get his factory 

 and the machinery ready. It has been the same with all 

 manufacturers and I do trust now that the necessities of 

 the workers have been placed clearly before the manu- 

 facturers that we shall soon reach a stage when we shall 

 have an instrument of our own manufacture, not only for 

 home use, but one which will also enable us to capture 

 the world's trade in microscopes. 



