64 SELECTION AND USE 



majority of continental makers adopt the opposite plan. 

 Their lenses are over- corrected, and objects show a bluish 

 border. 



A want of correction for color is shown when thin objects with 

 many cross lines are examined, especially with slightly oblique 

 light. As a test for achromatism in low powers, Carpenter 

 prefers a section of coniferous wood, showing the glandular 

 dots. He also recommends the tracheae of insects, but almost 

 any lined object will answer the purpose. 



The existence of aberration of form is best proved by the use 

 <)f a fine micrometer or a Nobert's plate. When this defect is 

 very marked, it is easily seen in the curved and distorted lines 

 of which the image consists, but such a state of things exists 

 only in extreme cases. Where this distortion is not very glar- 

 ing, it may be necessary to compare the magnified image of the 

 lines in the stage micrometer with straight lines ruled on a thin 

 plate of glass laid on the diaphragm of the eye-piece in other 

 words with an eye-piece micrometer in which the ruled lines are 

 quite long. 



For testing for flatness of field and aberration of form, Frey 

 recommends " a slide thickly smeared with India ink, in which 

 small circles or other figures are scratched with the point of a 

 fine needle. * * * If the instrument is adjusted with trans- 

 mitted light for such a circle, it should appear sharply 

 cut on the black ground, and not surrounded by a halo 

 of light. If the circle is then brought out of focus, it 

 gradually enlarges, while its sharp borders disappear, with- 

 out spreading a strong halo of light either inwards or outwards 

 over the black field." 



The angular aperture of an objective can be determined 

 accurately only by measurement, and this is something that 

 beginners will hardly attempt. To measure accurately the 

 angular aperture of an objective, is a task requiring con- 

 siderable skill and knowledge, and most of the appli- 

 ances furnished by microscope-makers for this purpose, fail 

 to give accurate results. It must be remembered that in 

 measuring the angle of an objective, we must comply with 

 the same rules that govern the accurate measurement of 

 any other angle. Dr. Carpenter, in his work, gives a method 



