PRACTICAL PHOTO-MICEOGRAPHY. 31 



< Working distance " is also apt to be curtailed by largeness 

 of angle. " Working distance " is simply the distance between 

 the object and the front combination of the objective. With 

 low powers this is of little moment, but where we come to use 

 high power objectives their performance is not only apt to be 

 impaired by very close working, but there is a danger of dam- 

 age to object or objective itself. Homogeneous immersion 

 helps us out of the trouble to some extent, but with cheap 

 immersion lenses of numerical aperture 1.25 and over, we have 

 repeatedly failed to observe objects that happened to have 

 cover glasses thicker than usual. 



The " power" of an objective is frequently very loosely 

 quoted. " Power " depends on focal length, and the focal 

 length of a compound lens is usually quoted by a supposititious 

 comparison with a single lens of given construction. The real 

 focal length of an objective, and consequently its amplifying 

 power, are very seldom accurately stated, even by the best 

 makers. If it is necessary to know the exact magnification of 

 any object with any objective at any distance, recourse must 

 be had to measurement by a stage or other micrometer. The 

 lower the power of an objective the more difficult it is to give 

 it wide angular aperture, consequently objectives which pre- 

 tend to wide angles are usually quoted under their real power, 

 i. #., over their real focal length. And tolerance of eye-piecing 

 is a very important factor in calculations as to power, for a 

 quarter-inch o. g. may stand an ocular of twice the power that a 

 one-sixth can bear, and so after all the one-fourth may come to 

 be the higher power. Well-made objectives will give good 

 images with oculars that will break down inferior lenses, and 

 a lens before purchase should always be tested with a high 

 ocular ; there is no better trial that can be rapidly made. 



There are certain proverbial tests for microscopic objectives, 

 and a glass is quoted as resolving this, that or the other test 

 structure. This is all very well if the tester be the owner of 

 the test object, and know it well. But opticians always have 

 test objects of their own, which, we need not say, are intended 

 for testing lenses to the satisfaction of would-be purchasers. 

 An optician's podura scale, or " blowfly's tongue," is generally 



