MECHANICAL ARRANGEMENTS. 61 



illuminated by an entire, or, as it may be termed, a solid 

 cone of light of much larger angle than that of the object- 

 glass. The extinction of an object by excess of illumina- 

 tion may be well illustrated by viewing with a one-inch 

 object-glass the Isthmia, illuminated by Gillett's condenser. 

 When this is in focus, and its full aperture open, the 

 markings above described are wholly invisible ; but as the 

 aperture is successively diminished by the revolving dia- 

 phragm, the object becomes more and more distinct, and 

 is perfectly defined when the aperture of the illuminating 

 pencil is reduced to about 20°. The same point may be 

 attained, although with much sacrifice of definition, by 

 gradually depressing the condenser, so that the rays may 

 diverge before they reach the object; and it may be 

 remarked, generally, that the definition of objects is 

 always most perfect when an illuminating pencil of suit- 

 able form is accurately adjusted to focus, that is, so that 

 the source of light and the plane of vision may be conju- 

 gate foci of the illuminator. If a condenser of 120° 

 aperture, or upwards, be used as an illuminator, the mark- 

 ings of Diatomacese will be scarcely distinguishable with 

 the best object-glass, the glare of the central rays over- 

 powering the structure of those that are more oblique. 



There are indeed almost as many methods employed 

 for bringing out the markings on test-objects, as there are 

 skilled and efficient microscopists, each one preferring his 

 own, simply because he has been at great pains in work- 

 ing it out with the utmost nicety. The late Professor 

 Quekett, in his treatise on the microscope, recommends 

 "oblique light with the mirror and lamp, removing all 

 appliances under the stage ; then, after much patience and 

 perseverance, Grammatophora subtilissima and the Amician 

 test may be resolved." Another plan is to use the flat 

 mirror, the achromatic condenser, and a paraffin lamp, day- 

 light not being so easily managed, or even so good for the 

 -|th or higher objectives. 



Mr. Lobb's method is seen in Fig. 36 : — "The microscope 

 is placed in the horizontal position, and a small camphine 

 lamp so adjusted that its reservoir may be close against 

 the end of the rack-tube ; having the A eye-piece, the 

 one -inch objective, and the achromatic condenser of 170 



