Experiments on Angular Aperiure. 37 



When the air in this interspace is rejolaced by water, the angle 

 becomes 100'^, or a Httle more. 



In this experiment the sUde and cover are thrown out as of 

 no importance to the solution of the question, viz. of the actual 

 angular dimension of the pencil traversing the object, and trans- 

 missible by the objective. 



It seems incontestable at all events that more than 82^ of 

 angular pencil can traverse the balsam-mounted object, and be 

 transmitted by the immersion objective to the eye of the observer. 



Incidentally to this proposition, the following is given when the 

 object is actually in situ and weU defined. 



Thus, instead of the water in the above experiment, human blood 

 was introduced between the plano-convex lens A, and the front 

 surface of the objective. 



As this necessitated, in order to bring the blood disks into view, 

 separating the systems of the objective (by means of the cover 

 adjustment) considerably, the apparent angle of this 170° objective, 

 i. e. the angle taken in the ordinary way, proved to be only 128°. 

 But the extreme i"ay transmitted when the blood was compressed 

 by the plano-convex lens upon the front sm'face of the objective, 

 proved to be less (a little) than 100°. 



This form, just detailed, of the immersion objective is a 

 " clinical " method, a year or two in use here, and wherein the front 

 surface of the objective becomes the stage of the microscope, a glass 

 " cover," or a lens as above, being applied to thin out the substance 

 viewed, be it blood, urine, or other material fit to be thus put under 

 view. 



A natural sequence of all this is the application of such a plano- 

 convex lens at the lower surface of the object shde, the primary 

 object of it being to avoid the excessive reflexion that takes place at 

 the immergent surface of an object-slide in all cases as now used. 



Of course, in such an objective as used in these experiments, 

 170° upwards, the pencil incident upon the immergent surface of 

 the slide must be to reach the full angle of the objective, very 

 nearly parallel with the face of the slide. Immense reflexion is 

 inevitable. 



The application of the plano-convex lens to the under immergent 

 surface of the object-slide allows the extreme incident loencil to 

 enter at a perpendicular incidence very nearly. To be sure the 

 convexity of the plano-convex lens has influence to modify this. 

 But by placing upon the plano-convex lens a plano-co»cave facet 

 lens (Fig. B), the incident rays meet a plaiie surface and pass on 

 to the object without suffraction. 



The increase of light in this latter case is necessarily large, and 

 the influence of that increase upon the result, i. e. the appearance 

 and demonstration of the object, is remarkable. 



