THE OBJECTIVE. 61 



maximum attainable resolving power of an objective of 

 180 aperture,, according to Professor Abbe, is tlie sepa- 

 ration or resolution of fine lines ruled 118000th of an 

 inch apart. Resolution depends more or less upon the 

 quality and quantity of the light admitted, the power 

 of collecting the greatest number of rays, and the per- 

 fection of centring. In other words, upon the co-ordi- 

 nation of the illuminating system of the microscope 

 mirror, achromatic condenser, objective and eye-piece. 

 If diatoms are employed as test- objects, it should be 

 borne in mind that there are great differences even in 

 the same species, and in the distances their lines are 

 apart. For this reason ruled lines of known value, as 

 Robert's lines, are much to be preferred. The follow- 

 ing example may be taken as a test of the value of a dry 

 Jth objective of 120 in defining the rulings of a 19-band 

 plate, which is equivalent to the l-67000th of an inch. 

 This objective, with careful illumination, showed them 

 all ; but when cut down by a diaphragm to 110, the 

 eighteenth line was not separable ; further cut down to 

 100 the seventeenth was the limit, to 80 the four- 

 teenth, and to 60 the tenth barely reached. 



Flatness of Field is a quality of some importance, 

 and must be included in the general practical value of 

 the objective, denoting its capacity to exhibit the peri- 

 pheral portions of the field with the same degree of 

 sharpness as the central. Flatness of field is much 

 enhanced by the width of the opening or angular aper- 

 ture. In all high -angled objectives the image should 

 be sharp and quite free from colour to the very mar- 

 ginal portions of the field. In experimenting on the 

 comparative merits of two object-glasses as to flatness 

 of field, an eye-piece of large aperture should be used. 

 For testing flatness of field, Cole's exquisitely prepared 

 double-stained sections of woods will be found in every 

 way suitable objects. The proboscis of the fly is also 

 recommended, and if its details have not been de- 

 stroyed by being mounted in balsam, it is a good test. 

 Glycerine is the proper medium for displaying its 

 several structures. The cover-glass of the object, it must 

 be remembered, as Amici pointed out, is not an nnimpor 



