150 SELECTION AND USE 



focus of the reflector; if, in adjusting the focus of the objective, 

 we move the reflector, we must, of course, destroy the proper 

 relation of the latter to the object. 



Objectives with Tapered Fronts. When the objec- 

 tive has a very broad front and a short working distance, as is 

 the case with most objectives of high angles, it is impossible 

 to illuminate the object satisfactorily with the bulls-eye con- 

 denser. To avoid this difficulty, several makers, notably Mr. 

 Swift, of London, have narrowed the brass work of their fronts 

 to the last limit, so as not to interfere with the illumination of 

 opaque objects. Mr. Tolles has, however, gone a step further, 

 and has allowed the glass of his front lenses to project beyond 

 the brass work, so that the bull's-eye may be used with com- 

 paratively high powers. We have a one-quarter constructed on 

 this principle, which allows of the clearest illumination, and 

 shows the P. angulatum as an opaque object so well that the 

 markings are sharp and well defined. These lenses are mode- 

 rate in price, very easily used, and give very satisfactory 

 results. 



Smith's Vertical Illuminator. This admirable de- 

 vice is due to Prof. Hamilton L. Smith, of Hobart College, Ge- 

 neva, N. Y., and is intended for use with objectives of such high 

 power, that the Lieberkulm, condensing lens, side reflector, 

 etc. , cannot be employed. Several different arrangements have 

 been suggested. The first was a small annular silver reflector, 

 placed just above the back lenses of the objective, and forming 

 an angle of 45 with the optic axis of the microscope. A hole 

 in the side of the brass mounting of the objective admitted the 

 light, which was thus thrown down through the lenses on to 

 the object, and back again to the eye. We have used such an 

 arrangement with most satisfactory results. For example, with 

 a one-fourth inch objective, thus fitted, it is easy to view the 

 P. angulatum as an opaque object, and bring out its markings. 

 The illuminator, as thus constructed, may be either a separate 

 reflector which may be screwed into the nose-piece of any mi- 

 croscope, and which is furnished at its lower end with the So- 

 ciety screw, into which the objective is inserted, or it may b 



