_The Webster-Collins Universal Condenser_ (Fig. 143) is so well known that it scarcely calls for any lengthy description. It is an inexpensive form of condenser, designed in the first instance for use with the students’ microscope. It is fitted into the sub-stage; has an iris diaphragm as well as a series of revolving diaphragms moved by a milled head screw arrangement.
[Illustration: Fig. 143.--The Webster-Collins Universal Condenser.]
_Wenham’s Parabolic Condenser._--Mr. Wenham’s many useful additions to the microscope and its accessories demand especial notice. When mention is made of the various immersion condensers (illuminators, as he preferred to call them), his original right-angled prism, his truncated hemispherical lens, his immersion paraboloid, and his reflex illuminator, in which rays beyond the angle of total reflexion are utilised by reflex action from cover-glass on to the surface of the object, every one of these well-devised inventions will always be spoken of in terms of praise. All in their turn conferred a great service upon the microscope, and enabled the student to clear up difficulties that stood in the way of developing structure when achromatic lenses and dry-objectives were considered perfect. The superior illumination of the object was wholly due to, and effected by, _reflected_ rays from the object to the aperture of the objective, and obviously, reflex action could only take place with dry-objectives. This reflex action must be regarded as Mr. Wenham’s special discovery. It must be observed, however, that it is not the same as the more modern achromatic appliances used for throwing _direct_ rays upon the object, and which proved the existence of apertures capable of direct transmission up to 27° measured in the body of the front lens.
[Illustration: Fig. 144.--Wenham’s Parabolic Reflector.]
The most practical of Mr. Wenham’s inventions is probably the hemispherical lens, since adopted by Messrs. Ross in connection with their excellent Zentmayer stand, and which has proved eminently serviceable. But the fact is that devices of the kind for obtaining direct oblique light require a thin stage, and therefore most of those who possess the earlier-made microscope stand would doubtless hail the appearance of any appliance which will convert axial light into oblique light; as by so doing the possessors of such instruments, in which the stage is generally of considerable thickness, would enjoy the pleasure of seeing the best resolution it is possible to get with their dry-objectives.[27]