302 Remarls on High-power Definition. ['^''^^i',^l\^j^^lZ'!^^] 



over the object, and demonstrated the nature of the aberration 

 caused thereby. We have here, in the separation or approximation 

 of the lenses of a microscope object-glass, an element of correction 

 ■which cannot be obtained in the telescope, and which, in constructing 

 the combination, enables us to neutralize the spherical aberrations 

 completely, and, to a great extent, without altering the radii. 

 Objectives, from the hands of careful and experienced makers, have 

 all been constructed on the globule test, and are not sent forth till 

 every en"or of workmanship, centering, state of oblique pencils, 

 achromatism, and spherical aberration — are obsolutely corrected ; for 

 this test discovers the least fault in either, where all others will fail. 



But in viewing difficult test-objects with the highest powers, one 

 source of error may occur from the following cause : — If a large 

 angular pencil of rays converging to a focus is transmitted through 

 a parallel plate of glass, an approximation to the form of spherical 

 aberration is produced of a negative character, viz. the marginal 

 rays are thrown beyond the central ones. It may easily be demon- 

 strated that this is not exactly identical in form and character with 

 the positive aberration caused by a lens with a spherical surface, 

 and that the operation of bringing the lenses of the microscope 

 object-glass nearer together for the counter-correction, will only 

 neutralize the error within certain limits. The irregular position 

 assigned to the marginal rays by a very thick plate of glass, cannot 

 be exactly reformed by the opposite error caused by closing the 

 lenses ; and it is a well-known fact to those experienced in the 

 resolution of test-objects, that some of the same specimens are 

 defined better under one thickness of covering-glass than another. 



In the front lens of an object-glass, tliiehness is a very important 

 element of correction. I have explained this in my paper " On the 

 Construction of Object-glasses ; " * and in working out a new combi- 

 nation it may be necessary to make several fronts in order to 

 arrive at the exact gauge. If an ordinary " dry " object-glass, 

 perfectly corrected, with a proper thickness of the front lens, be 

 used as a so-termed " immersion " lens by the introduction of water 

 between the front lens and covering-glass, this immediately becomes 

 a part thereof, and the excess of negative aberration, both spherical 

 and chromatic, is not to be corrected by the usual separation of the 

 lenses. The whole combination has become over-corrected. Kays, 

 which before the introduction of water emerged from the upper 

 surface of the plate in a line parallel to their first incident direction, 

 now pass on in a nearly straight course from their primary refraction 

 from the under-surface of the cover. In order, therefore, to employ 

 an objective as an immersion lens, it becomes requisite to have a 

 thinner front, all other radii and corrections remaining the same. 

 The extra or immersion lens should have its thickness diminished by 

 ♦ Published consecutively in the earlv numbers of this Journal. 



