324 SUMMAIIY OF CURRENT RKSE.\.RCnES RELATING TO 



mcasuriug the powers of oculars prove impracticable or undesirable, the 

 cftbrt may at least lead to the discussion and adoption in some form 

 of the nomenclature by inches, which is the essential portion of the 

 proposition. It remains for the Society to say whether it is prepared 

 to take the responsibility of trying the experiment or not." 



Testing the different Sectors of Objectives.* — Mr. E. M. Nelson 

 finds that all the sectors of an object-glass are not equally good in 

 defining objects. In fig. 76, sectors 1 and 2 may not bo as good as 

 3 and 4. If, however, to test this we rotate tho object, the test is not 

 a satisfactory one, as tho illuminating conditions are 

 Fig. 76. altered, so that when, for instance, a Podura scale is 

 being rotated, something must be allowed for the altera- 

 tion of the illuminating conditions with 'regard to the 

 position of the exclamation marks, as well as something 

 for the difference in the quality of the sectors. 



" The importance of separating these variables will 

 be obvious to every one. For this purpose I have de- 

 signed a revolving nose-piece, which will enable the 

 object-glass to be turned round, and so bring its various sectors into 

 play. By this means it can be easily demonstrated how much of the 

 difierence in the pictures is due to the objective, and how much to the 

 illumination. \\ hen I practically tried this nose-piece, I was very 

 much astonished at the enormous diflference I found in the defining 

 powers of the alternate sectors of an object-glass. To illustrate the 

 difference in the chromatic aberration, let me mention only an ex- 

 ample. A very fair water-immersion 1/8, 1 • 17 N.A., showed the 

 exclamation marks red in one position, but turned them green when 

 the object-glass was rotated through an angle of about 90°. 



" The practical outcome of all this is important. (1) An object- 

 glass which performs well enough when exhibited on the optician's 

 Microscope, may tell a very different tale when tried on the purchaser's 

 instrimaent, because the objective may not screw up to the same point. 

 (2) It may account for the difference of opinion held by experts as 

 to the quality of any particular objective, for they might have been 

 testing different sectors. 



" It would be worth while, in the case of expensive object-glasses, 

 to have the Society's screw portion of them capable of rotation, and 

 provided with jam screws, so that the purchaser might place the 

 better pair of sectors in a line across his own Microscope, then fix it 

 with the jam screws. He would only have to remember to place any 

 exceptionally difficult object in a line with the front and back of his 

 stand." 



Mr. Nelson was no doubt not aware when he wrote the foregoing, 

 that the testing of objectives in the way suggested has been practised 

 for many years. All tho objectives of Zeiss are thus tested. The 

 matter is referred to by Dr. Dippel, from the suggestions of Prof. 

 Abbe, as follows : f — 



To ascertain the faults which arise from defective centering, or 

 which act in a similar way, and may be called want of symmetry in the 



* Engl. ]\Iccli., xli. (1885) p. 34 (1 fig.). 



t See Dippel'a ' Das Mikioskop,' 1882, pp. 347-8. 



