ADJOURNED DISCUSSION IN LONDON 241 



one after; the other with the same object glass were fairly consistent, 

 which, considering the conditions under which these observations 

 are made, is rather remarkable, because one is using an extremely 

 small portion of the object glass at one time. The principle is that 

 by the use of a small diaphragm you are illuminating a small zone 

 of the object glass, and the numerical aperture of the portion you 

 are illuminating is very small. I did not expect that our results 

 would agree, because of the extremely inferior image produced with 

 such a small portion of the object glass being used at one time. In 

 discussing this matter with Dr. Hartridge, he pointed out that his 

 microscope was not nearly sufficiently rigid for the purpose. The 

 matter has been considered by my firm, and they came to the con- 

 clusion that there was no microscope sufficiently rigid for the pur- 

 pose, and consequently for the last eight weeks we have been design- 

 ing an instrument which I am proposing to make for my own personal 

 use that I hope and think will be the most perfect microscope stand 

 ever made. I shall show it to the Society as soon as it is made. 

 Those who use the microscope for general work may consider 

 it too elaborate and expensive for ordinary purposes, but I am not 

 sure. It will have some features about it that will make it unusually 

 rigid. Its construction is an interesting engineering problem, and 

 whether anybody will ever order a similar one may be doubtful, 

 because the cost will be very great. 



There is one point made by Dr. Hartridge in his paper which 

 I think is an obvious error, and if it were pointed out I think he 

 would admit it. The method of testing the object glass is only a 

 test to see whether the light from a lens is going to one point. It 

 is not a test of the sine condition. That must be carried out as a 

 separate test, and I am bound to say that my own impression is 

 that when the Hartridge test is worked out and his method of cali- 

 brating and plotting out has been done, we shall find we are testing 

 an important, but not by any means the most important, correction 

 on an object glass. The important points about an object glass, 

 apart from achromatic corrections, are firstly, that the light from 

 the whole object glass shall go on to a point, and secondly, that 

 the focal length of every zone in the glass shall be the same, and 

 it is this latter point that the sine condition guarantees. Mr. Hart>- 

 ridge's test has some analogy to the Hartmann test; it measures the 

 lateral shift of the uncorrected rays instead of the longitudinal error. 



Commander M. A. Ainslie, R.N.: I should like to concentrate 

 attention on the subject of the condenser. Professor Conrady refers 

 to the incorrect position of the iris diaphragm; this is certainly 

 most marked, but there are one or two points to be considered in 

 this connection. There is no reason why the iris diaphragm should 

 not be placed between the top lens of the condenser and the next 

 lens, or perhaps a little lower down; at any rate, much higher up 

 that it is at present. The diaphragm could be very well worked by 

 means of a bevel wheel and a pinion coming out radially; the only 

 thing against this is that the stage is so thick. It would be quite 

 impossible on the standard instrument here shown, but if we were 

 to return to the " horseshoe " type of stage designed by Nelson, 

 it could be done perfectly easily. Presumably, however, the exit 

 pupil of the objective is in the neighbourhood of its upper focal 



