MEDIUM-POWEH PHOTO-MTCROGKAPHY 



85 



secured by a touch of a cover-glass adjuster, so they provide their immersion 

 objectives with them. Great care must be exercised in using all immersion lenses 

 for they are delicately made, especially as the power increases, by which is meant 

 that the greater the N. A. the more care must be used. This is made obvious by 

 the study of Fig. 46 : on the right is an objective with N. A. i "30 and the left 

 one of I '40 (after Zeiss.) 



Dry lenses which are not provided with a cover-glass adjustment have to be 

 corrected by pushing in or pulling out of the draw-tube of the microscope, which 

 optically amounts to 



^ Fig. 46 



almost the same as the 



use of the cover-glass 

 adjuster, lengthefiing the 

 tube with thin cover- 

 glasses and shortening 

 it with thick ones. 



The careful reader 

 will now very readily 



understand what is meant by modern objectives having written upon them very 

 plainly — "For 170 mm. tube" or perhaps " Tubuscule 250 mm.," and so on — it is 

 the length of tube for whicli the objective has been originally corrected with a given 

 thickness of cover-glass. Each maker has his standard thickness of cover-glass, and 

 this can be furnished on application. With low powers all these minutiae vanish to 

 a great extent, but when dealing with high -power dry objectives they become of 

 the most paramount importance if perfection of result he aimed at iii the filial 

 photograph. 



(ii) Numerical Aperture.— This term has often been used, and its usual 

 expression is by the letters N. A. The N. A. of a microscopical lens bears the same 

 relation to its performance as the aperture of a telescope does to that instrument. 

 Astronomers full well know that the larger the object-glass becomes, the greater 

 power the instrument possesses for separating close " double-stars." To put it another 

 way, some stars which with a small telescope appear as single are easily separated 

 into " doubles " when sufficient aperture is employed. Increasing the diameter of their 

 object-glass yet more, perhaps one of the " doubles " itself, is found to consist of two, 

 and it is no hidden fact that the higher the diameter of the telescopic objective the 

 more " double stars " are found to exist. This is not the place to discourse upon the 



