28 JOURNAL OF THE [February, 



A SIMPLE AND INEXPENSIVE FORM OF BLACK- 

 GROUND ILLUMINATOR. 



BY PROF, ALFRED M. MAYER, OF THE STEVENS INSTI- 

 TUTE OF TECHNOLOGY. 

 {Read Jan. 15M, 1886.) 



This is a simple and inexpensive form of black-ground illumi- 

 nator, devised for the study of aquatic life with low-power ob- 

 jectives of angular apertures up to 50° or 60°. For this purpose 

 it works admirably, showing aquatic organisms as brilliant ob- 

 jects on a black ground, so that they are instantly detected 

 among the more opaque particles of ooze in which they are gen- 

 erally found ; thus saving much time in such studies. 



The illuminator shows these objects in their true colors ; for 

 the pencil of illuminating rays, when properly adjusted, is sen- 

 sibly colorless. The interior structure of rhizopods, infusoria, 

 rotifers, worms, &c., is brought out in a manner which is very 

 striking. 



An angular aperture far exceeding that of the objective is not, 

 in my opinion, desirable. This opinion is based on experiments 

 with other dark-ground illuminators which give these large 

 angles to the emergent pencil. With these we do not see so well 

 the interior structure of translucent bodies — probably by reason 

 of the interior reflections produced on rays falling at angles of 

 too great incidence (when referred to the axis of the lens) on 

 the boundaries of those portions of the organism which have 

 different refractive indices. It is my opinion that, if the rays 

 enter the objective at the smallest angle required for a black- 

 ground, the interior structure of these bodies is shown in the 

 best manner. Objects exhibited by this illuminator show no 

 glow around their borders. A glow is often seen when other 

 illuminators are used, and interferes much with good definition 

 of the margins of objects. 



The light which was used in my illuminator is obtained by 

 placing a hollow lens, of the diameter of about five inches, be- 

 tween the flame of a student's-lamp and the plane mirror of the 

 microscope. The fiame of the lamp is diaphragmed down to a 



