22 Ohjeci-glasses and their Definition. 



distinct image with the excessive aberrations both chromatic and 

 spherical purposely introduced in the back combinations, to be 

 afterwards neutraHzed or corrected by the counter-aberrations of 

 the front. In this way the globule is used throughout the series ; 

 I know of no substitute ; and so reliable and comprehensible are its 

 indications to the worker of lenses, that I have no hesitation in re- 

 peating this, my former assertion, commented upon by Br. Pigott, 

 " that without this test it would be impossible to construct perfect 

 objectives." That the separation of points of light on the globule 

 coming from two sources placed at a given interval is a test, no one 

 will question. A bad glass will blur them together, and a good one 

 will separate them more or less distinctly. Andrew Eoss employed 

 this method of separating known intervals, in his experiments on 

 the defining power due to aperture. Dr. Goring used the reflected 

 image of the bars of a window, as explained in " Micrographia ;" 

 and the globule-reflected image of a Venetian bhnd, adjusted so as 

 to regulate the width of the dark and light intervals, also makes an 

 excellent test ; or the wire gauze and perforated zinc employed by 

 Mr. Lister. 



As again quoted by Dr. Pigott, I have stated in reference to 

 the immersion lens that " not either the water or glass cover has 

 introduced a single new element of correction," meaning that these 

 are combined identically as part of the front lens. This assertion 

 may have appeared dogmatical, but was based upon practice. My 

 experiments in the construction of object-glasses have been conducted 

 more with the view of originating new combinations than repeating 

 the same thing. The single fronts were made very thin to begin 

 with, and the measure for the necessary thickness for correction was 

 arrived at by means of different thicknesses of glass stuck on the 

 front with Canada balsam, but more frequently by water ; the dimen- 

 sion thus obtained was employed for finally making another front of 

 the correct thickness. The object-glasses were strictly immersion 

 lenses at this stage of their existence. I did not then think it worth 

 while to retain a special immersion object-glass (i. e. one with a thinner 

 front), particularly as several of the covers of my test object were 

 cracked and could not therefore be used with water ; but as some 

 further proof of the particulars and dimensions are now needed, I 

 have recently removed the front of a y^th and substituted a thinner 

 one of the same curvature to be used as an immersion. Fig. 2 is 

 the dry lens fifty times the size of the original ; the rays are taken 

 through from beyond the glass cover, with the object-glass corrected 

 for this thickness, which combines as part of the lens. Fig. 3 is 

 the immersion front, wherein the lens, water, and cover form the 

 unity, also corrected on the object for thickness. The aperture or 

 angular pencil passing through the combination from a dry-mounted 

 test is the same in both cases. In Fig. 2 the thickness of the lens 



