54 



THE AMERICAN MONTHLY 



[March, 



being allowed to divide the field 

 equally at each extreme of the angle. 

 In case the measurement is to be 

 made by the aid of the substage bar, 

 this may be made to carry a small 

 candle or lamp, which should be 

 diaphragmed to a narrow slit. The 

 measurement, as before stated, should 

 be made from the point at which the 

 light bisects the field. When it is 

 desired to measure balsam aperture, 

 or the angle of the rays in Canada 

 balsam, it is necessary to use the 

 proper immersion fluid, and to have 

 a hemispherical lens placed under the 

 object (the object, of course, being 

 balsam mounted) whose centre of 

 curvature is in the plane of the ob- 

 ject. The readings upon the base or 

 substage bar then indicate the angle 

 within the hemispherical lens ; and if 

 this be of the same glass as the front 

 lens of the objective, the true glass 

 angle, or, as it is . more commonly 

 called, the balsam angle, becomes 

 known. 



I think it may not be unjust to say 

 that many of the objectives of our 

 best makers, will fall considerably 

 short of the aperture claimed for 

 them. A recent ^ for which an angle 

 of 130° in balsam, was claimed, only 

 measured 125°; a very fine ^V for 

 which 140^ was claimed, only gave 

 116°; and a iV claimed to have 105°, 

 only gave 97° 



An objective may have a very wide 

 angle of aperture and still be inferior 

 to that of another objective of less 

 angle. In such cases the trouble is 

 generally due to a want of proper 

 correction for chromatic and spheri- 

 cal aberration. In measuring aper- 

 ture it is always well to select some 

 test-object which shall be more or 

 less difficult to resolve with the ob- 

 jective to be measured. It should 

 then be focussed and carefully 

 watched as the stand or mirror bar 

 is turned, until the very best resolu- 

 tion is attained. By increasing the 

 obliquity of the light it may then be 

 seen whether the extreme rays which 

 traverse the periphery of the objec- 



tive are really of any use. For ex- 

 ample, in the i already mentioned 

 the light will enter the objective at 

 125°, but the light which forms the 

 image only enters at 120°; hence the 

 objective is no better than one prop- 

 erly corrected having 120°. 



Flatness of field and penetration 

 are two qualities of which I can say 

 but little. Many of the best objec- 

 tives have a very flat field, but I have 

 never yet seen one in which the defi- 

 nition was really good to the extreme 

 edge. It is easily determined by ex- 

 amining any flat, well-marked object, 

 such as a section of echinus spine 

 with the lower powers, and a slide of 

 evenly spread blood-corpuscles with 

 the higher ones. 



Working distance is of especial 

 value in high powers. It is the dis- 

 tance of the object from the front 

 lens when in focus. It can only be 

 ascertained by actual measurement. 

 For this purpose a scale and vernier 

 on the ways of the body are very de- 

 sirable, but I have never yet seen a 

 micrometer screw sufficiently good to 

 correctly measure the working dis- 

 tance of the higher powers. The 

 best way of measuring the greatest 

 working distance is by having a num- 

 ber of cover glasses of various thick- 

 nesses, accurately measured, and see 

 which is the thickest that the objec- 

 tive can be made to focus through, 

 using the ten-inch body. It will be 

 found that the actual distance be- 

 tween the object and the objective 

 will increase with an increase in 

 thickness of cover-glass or the density 

 of immersion fluid. 



The magnifying power of the ob- 

 jective is a matter well worth know- 

 ing. Many objectives are underrated ; 

 for example, a recent -jV which I 

 measured only gave 84 diameters 

 as its greatest power, while another 

 iV g^ve the unusual power of 120 di- 

 ameters, thereby being a true -jV. 



The correct point from which to 

 measure the power, is the optical 

 centre, but as this changes with every 

 change of immersion fluid or ienjrth 



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