2 ELEMENTARY CHEMICAL MICROSCOPY 



whose principal focus lies at the distance marked upon the ob- 

 jective. 



In a similarly constructed series, the smaller the value of the 

 equivalent focus, the greater will be the magnifying power of the 

 objective. A few manufacturers still arbitrarily letter or number 

 their objectives. In such cases it is generally the rule that the 

 earlier in the alphabet the letter or the smaller the number in 

 the series the lower the magnifying power. 



When properly focused upon a preparation, the front or lowest 

 lens entering into the construction of an objective is usually 

 nearer to the preparation (in dry objectives) than the distance 

 indicated by the equivalent focus. This distance between the 

 front combination of the objective and the preparation, when in 

 focus, is known as the working distance of an objective. In their 

 selection for use in microchemical analysis the working distance 

 becomes one of the most important considerations affecting the 

 choice of the objectives. 



The construction of typical microscope objectives is shown 

 diagrammatically in Figs. 17, 1 8 and 19, page 44. 



All objectives are corrected to a greater or lesser degree for 

 chromatic aberration (presence of colored fringes around the 

 images) and also largely for spherical aberration (failure to yield 

 a flat field of view) . When the spherical aberration is so corrected 

 as to yield an especially large and flat field the objectives are 

 often called aplanatic objectives. Although an objective may 

 be so corrected as to yield a flat field, images of objects lying near 

 the circumference are apt to be hazy or indistinct, the result of 

 a form of spherical aberration known as coma; this is especially 

 marked in high power objectives and requires unusual care in 

 construction for its elimination. 1 



In all ordinary so-called achromatic objectives the corrections 

 are usually such as to bring the rays of two spectral colors to a 

 focus. In such lenses the optical and chemical foci may lie in 

 different planes and therefore such objectives may not give 

 really good results if employed in photomicrography; for this 

 reason specially corrected achromatic lenses called photo- 



1 See Spitta, Microscopy, London, 1909. 



