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On a " False-Light Excluder " for Objectives. 



By John E. Ingpen. 



(Read April 2Uh, 1874.; 



I wish to bring before your notice this evening a simple method 

 of getting rid of the glare frequently present even in the best 

 objectives. When I tell you that it is nothing but a cap having a 

 small aperture, and capable of being slipped over the objective, you 

 will probably consider it a very insignificant affair ; but the principle 

 involved is an important one, and well worthy of a few minutes' 

 attention. 



In the *' Monthly Microscopical Journal " for March last, p. 112, 

 there is a letter by Mr. Wenham, in which he explains his method 

 of measuring the angle of a Tolles' i-objective belonging to Mr. 

 Crisp. The principle of this method is that the rays which con- 

 stitute the true working aperture must be " image-forming rays 

 " only." Mr. Wenham contrives to admit only those rays which 

 can enter an aperture the size of the field of view, and in the plane 

 of the object ; and he shows how false light may enter an objective 

 outside the field of view, and at a much greater angle than any 

 rays which could possibly form an image of the object ; and he 

 proves that the true working angle of the Tolles' 1 cannot be 

 greater than 118*^, while stray light can enter the front lens almost 

 up to 180^ 



Upon reading the above letter, it struck me that the method by 

 which false light was excluded in measuring the angle would 

 equally well serve to keep out false light in using the objective, 

 and thereby improve the definition. I had not to go far to apply 

 it. A fine Eoss \ of 100^, with which I constantly worked, always 

 shewed a blaze in the centre of the field when direct light was used, 

 while with oblique light its resolving power was excellent. I soon 

 had a cap made with an aperture a little larger than the field of 

 view with the lowest eye-piece ; when this cap was slipped on to 

 the objective, and brought down so as to touch the covering-glass, 

 all the false light vanished, the milky appearance of diatoms so 

 often seen when viewed with large angled glasses was replaced by 

 a clear black definition, while the angle was scarcely diminished, 



