382 OBJECTIVES, EYE-PIECES, THE APERTOMETER 



reader that many of the more important qualities of objectives 

 are determined by the principles of their construction, and become 

 in fact questions simply of the quality of the workmanship involved 

 in producing the optical and mechanical parts of the object-glass. 



The quality of the workmanship may be tested by technical 

 means described below, and by that subtle power which comes with 

 experience. This can only be imparted through the paths of labour 

 and experiment, by which in every case it is reached. But, granted 

 that an object has been illuminated in an intelligent and satisfactory 

 manner, the first complete view of the image (which must of course 

 be a thoroughly familiar one) will enable the expert to come to a 

 conclusion as to the quality of a given objective. The character of 

 the image to the expert determines at once the character of the 

 lens. This is the more absolute if a series of eye-pieces (up to the 

 most powerful that can be obtained) are at hand. Nothing tests 

 the quality of an objective so uncompromisingly as a deep eye-piece. 

 For brilliancy of image a moderate power of eye-piece is of course 

 best ; but the capacity of the object-glass is clearly commensurate 

 with its ability to endure high eye-pieces without loss of character, 

 and even sharpness in the image. Unless the objective be of high 

 quality, the sharpness of the image gradually disappears as the more 

 powerful eye-pieces are used, until at last either all or part of the 

 image breaks up into the 'rotten' details of a coarse lithograph. 



A lens finely corrected (w^ith large aperture) will bear the deepest 

 eye-piecing with no detriment. The 24 mm. and the 12 mm. of Zeiss 

 will suffer any eye-piecing accessible to the microscopist without the 

 smallest surrender of the sharpness of the image. We have in fact 

 tried in vain to ' break down the image ' yielded by these objectives. 

 This mode of testing is of course to a large extent subjective, or 

 at least is controlled by incommunicable judgments. It is most 

 important therefore to have a mode of judgment that shall be acces- 

 sible to the beginner and the interested amateur. Dr. Abbe has 

 proposed a method which is at least accessible to all. 



In ordinary practice microscope objectives, if tested at all by 

 their possessors, are simply subjected to a comparison of perform- 

 ance with other lenses tried upon the same 'test-objects.' 



The relative excellence of the image seen through each lens may, 

 however, depend in a great part upon fortunate illumination, and 

 not a little upon the experience and manipulative skill of the ob- 

 server ; besides which any trustworthy estimate of the performance 

 of the lens under examination involves the consideration of a suit- 

 able test-object, as well as the magnifying power and aperture of the 

 objective. It is knowing what is meant by a ; critical image,' and 

 being able to discover whether or not a given objective will yield it. 

 Clearly all tests of optical instruments, which are not capable of 

 numerical expression, must be comparative. Magnifying power can 

 be measured numerically ; it is not comparative. In the same way 

 resolving power is mathematically measurable ; so is penetrating 

 power. But definition and brilliancy of image, and evidence of 

 centring, can have no numerical expression ; they are consequently 

 comparative. 



