72 Modern Microscopy 



The Abbe Test-Plate. 



The most satisfactory way of testing an objective that is 

 at the disposal of him who would learn the whole inward- 

 ness of his lens is the Abbe test-plate. A considerable 

 amount of experience will be required to use it advan- 

 tageously, but it discloses at once, when employed by one 

 who has learnt to appreciate its significance, any mechanical 

 inaccuracies that may exist. Directions for using accom- 

 pany each plate, but the worker will quickly mark out for 

 himself a line which experience will show him is the best 

 for quickly ascertaining whether the lens is accurately 

 centred and the state of the corrections for spherical and 

 chromatic aberrations. The test-plate itself consists of six 



FIG. 19. 



discs of cover-glass, all of different specified thicknesses 

 and embracing such a range as objectives are likely to be 

 corrected for. 



On the under-surface, lines are ruled in a deposit of silver, 

 and the covers are mounted on an ordinary 3x1 slip. The 

 ruled lines are coarse, and can be separated with a low- 

 power objective. The procedure adopted by the writer is 

 as follows : 



The tube -length should be that for which the objective is 

 ostensibly corrected. An eyepiece of high power and a sub- 

 stage condenser, giving a solid cone equal to at least two- 

 thirds the total aperture of the objective, are used. 



In this connection it may be mentioned that it will be 

 found advantageous to have an eyepiece which permits of 

 some degree of over or under correction being obtained 



