THE PRESIDENT'S ADDRESS. 151 



use of fluorite (which I might add enters into the formation of 

 the true apochromatic) does not constitute such objective a true 

 apochromatic. Indeed, there are some sold under this name 

 which do not equal a good semi-apochromatic. 



As most of these objectives necessitate the use of a special 

 form of ocular it may be well here to describe its function, for I 

 find it is often misunderstood, especially as the use of compensating 

 eye-pieces certainly improves the performance of the better class 

 of semi-apochromatics almost to the same amount as it does with 

 the true apochromatics. 



To explain the use of these compensating eye-pieces, let us first 

 try a high-power apochromatic with an ordinary Huygenian 

 ocular, using white light with an object of strong contrast such 

 as may be furnished by an Abbe test-plate. We find, perhaps 

 to our surprise, excellent definition in the centre of the field, but 

 an immense amount of primary colour (that is, yellow and blue) 

 in the marginal parts of the field. ]S"o wonder, therefore, that 

 even some experienced microscopists are led to think that the 

 compensating ocular plays a very large part in the colour 

 correction of this modern type of objective. But if, retaining the 

 same ordinary ocular, we substitute monochromatic light of various 

 colours in succession, we find, again perhaps to our astonishment, 

 that without alteration of focus (or, at any rate, with but a very 

 slight one in the case of the semi-apochromatic), any one colour 

 gives extremely fine and sharp definition all over the field of 

 view ; but that the different coloured images are of different 

 magnitude — red, for example, producing considerably lower 

 magnification than blue or violet. This peculiarity is due to the 

 thick unachromatic front lens found in all these modern con- 

 structions, which causes them to have a shorter equivalent focus 

 for blue than for red rays as before explained. A compensating 

 ocular, then, is merely one which has a corresponding chromatic 

 difference of magnification but in the opposite direction, although 

 of the same amount, so that the final magnification is the same 

 for all colours. This chromatic difference of magnification is, 

 then, the only aberration that the compensating eye-piece can or 

 does correct : all the other corrections must be effected by the 

 objective itself. 



In conclusion, it may be of interest to tabulate the number 

 of conditions which the objectives of various degrees of perfection 



