70 JOURNAL OF THE [October, 



3. Penetratinff power. 



4. Illuminating power. 



5. Flatness of field. 



6. Defining power. 



" I. No test is required for this, as it can be directly measured. 

 Imagine anyone saying that a scale of Morpho Menclaus was a 

 good test of magnifying power ! 



" 2. Resolving power is simply numerical aperture, or N. A. 

 This can also be directly measured, therefore no test is necessary. 



" 3. Penetrating power is the reciprocal of resolving power, 



; no test is necessary. 



N.A.' ^ 



"4. Illuminating power is (iV. ^.)^, or N. A. multiplied by 

 itself ; no test is necessary. 



"5. Flatness of field. I'ests : for low jiowers, a microphoto- 

 graph ; for medium powers, a stage micrometer ; for high powers, 

 a slide of minute bacteria or micrococci, dried on cover and 

 stained. This is not so important as usually supposed, 

 especially in high powers. 



" 6. Defining power depends, as we have seen, on the freedom 

 of the lens from spherical and chromatic aberrations. Of these 

 two, the spherical is the all-important one. 



"Of the method of testing lenses for this point, I have treated 

 already at length in these columns, and therefore will merely 

 say that it is performed by viewing a suitable object illuminated 

 by solid right cones of light, the object being placed in the apex 

 of the cone. Cones of small angles should first be used, and 

 then enlarged until the object begins to get pale, milky, or 

 foggy. On removing the eye-piece and looking down the tube 

 at the back lens of the objective, it will be seen what portion of 

 the lens is filled with light, thereby determining at what point in 

 the aperture of the objective the spherical aberration begins to 

 operate, the best lens being that which will stand the most light. 

 Flooding an object with too much light is only another name 

 for spherical aberration in the object-glass. A good objective 

 is one which cannot be flooded. 



" Suitable tests for spherical aberration : 



"Very low powers, 3, 2, i^-in. ; wing oi Agrion pulchellum ? 

 (Dragon-fly). 



"Low powers, i, I in.; proboscis of Blow-fly, squeezed flat. 



