Oil Zeiss -isth Immersion. By W. J. Hickie. 185 



of the former 664^. This excess of angle is a false quantity, 

 because it does not come as a radiant from one single position in the 

 axial focal point, but from other lateral rays of the marginal foci. 



From this it may be inferred that unless precautions have been 

 taken to exclude these lateral pencils, all measurements for ascer- 

 taining large apertures have hitherto been erroneous, and far in 

 excess of the true pencil from a single radiant point ; and this source 

 of error will exist from the same cause, whether the measurement 

 is taken by means of the usual convenient sector, or what may be 

 termed the telescope method, of obtaining distinct distant images 

 with a suitable eye-j)iece arrangement. 



I disclaim invidious intentions in putting forth the expression, 

 that the apertures of high-power object-glasses published in the lists 

 of opticians are all wrong, and shall not quote any particular one 

 as an example. If this is a scientific fact, let it be fairly discussed 

 as such, without rancour or personality. The question is a simple 

 one, and I cannot at present see that I am in error in a theory 

 clearly proved by point of fact measurement. 



The result may always be verified by the measurement of the 

 focal distance (suitably corrected by the adjustment) and size of 

 the working portion of the front lens, through which rays are 

 admitted. Many will be surprised to find how small this portion 

 is. In high powers it may be defined by screwing the object-glass 

 in the place of the achromatic condenser, and the diameter of the 

 spot measured with a micrometer, used with a low power. The 

 front of the object-glass should have a drop of milk dried upon it 

 to serve as a screen, or if the lens is flush with the setting, a piece 

 of thin covering glass, greyed on one side, may be laid upon it, by 

 means of which the proper light area can be determined having 

 a definite margin. 



V. — On Zeiss' wzfh Immersion. By W. J. HickIe, M.A. 



About the 1 8th of last October I obtained from Zeiss, of Jena, one 

 of his ^Vths (No. 3 immersion), and during the first week in 

 January the same maker forwarded two others, which he had 

 completed only a few days before. On the evening of the same 

 day all three were submitted to a trial of their capabilities. In the 

 account I am about to give, I shall indicate the three w^ths then 

 examined by the letters A, B, C, where A denotes the objective 

 first sent, while those sent in January are denoted by the letters 

 B and C. As A had been in constant use for more than two 

 months, I had had ample opportunity of making myself acquainted 

 with all the peculiarities of this description of lens, as also with the 



