'^omnulST.T'Ta] Remarh on High-power Definition. 801 



as I find it recorded in the Journal, in a fair spirit of controversy, 

 being willing either to receive or give any information that may 

 tend to elucidate the truth. 



In the first place, I must take some exception to the slur that 

 is cast upon the object-glasses of our best makers, by the assertion 

 that " in the best glasses there is a certain residuary aberration, 

 which obscures the clear definition under a power of 1000." If 

 such an error does not exist, of course all mathematical calculations 

 for demonstrating its character and amount must be in vain. 



The high-power objectives, from ith upwards, constructed by 

 our first-class makers during the last fifteen years, may no\^ be 

 named as hundreds. Surely some of these are absolutely perfect, if 

 not the majority ; and if any error should be present, the develop- 

 ment of a peculiar structure in a test-object is not a certain way of 

 detecting it. In this inquiry, it is remarkable how the use of the 

 mercury globule is ignored ; yet I have no hesitation in saying that 

 without this test it would be impossible to construct perfect 

 objectives. To the practised eye of the microscope optician, it will 

 develop errors that can be detected by no other means. With a 

 good |th, for example, under perfect adjustment the spherule 

 appears clear and bright, with the reflexion of surrounding objects 

 shown thereon ; and the only fault is that arising from the 

 secondary spectrum, seen as a pale-green halo beyond the focus. 

 It would be desirable to correct or diminish this, but the cure lies 

 more in the hands of the glass maker than the optician. When 

 the globule is thus perfectly defined, if the least possible touch be 

 given to the adjusting collar, altering the distance between the 

 lenses by something less than roVoth of an inch, a kind of fog mars 

 its brilliancy, and is the result of spherical aberration, positive or 

 negative, accordingly as the front lens is either separated or brought 

 nearer. Objects seen by transmitted light are most uncertain tests 

 for these errors of aberration. 



If an object-glass is adjusted by a Diatom, or Foclura, viewed 

 by transmitted light, and this same object then illuminated on a 

 dark field, it will generally be found that the first adjustment was 

 imperfect, as a fog now oftentimes obscures the object, which is 

 dispelled by further and more careful adjustment, with the more 

 sensitive test of opaque illumination. Thus in the best objectives 

 we have the power of obtaining equally both positive and negative 

 aberration, and the position between them is free from either — 

 supposing that there are no errors of workmanship. These being 

 under the control of the artist do not frequently occur, and cannot 

 be classed as a constant error. 



Object-glasses were made eighteen or twenty years ago with 

 smaller apertures, giving as perfect definition as now. Andrew 

 Eoss discovered the adjustment for the thickness of glass-cover 



