The Microscope. 241 



occur. Air bubbles and oil drops both appear with a bright 

 central spot and a broad dark border. To obtain the former 

 Prof Gage suggests that a drop of mucilage shall be placed on a 

 glass slip and beaten with a broad blade until it looks milky on 

 account of the inclusion of air. Put on a cover glass but do not 

 press it down. With this under the objective, focus upward and 

 downward, noticing that in focussing up, the central bright spot 

 becomes very clear and the black ring very sharply defined, un- 

 til the whole is dimmed by being far beyond the focus. As Prof 

 Gage also says, the air bubble is one of the most useful means 

 for ascertaining whether or not the illumination is strictly cen- 

 tral ; if it is not, the bright spot will not be in the centre of the 

 bubble, as it will be if the light is strictly axial. And if the 

 mirror be swung to one side so as to make the illumination ob- 

 lique, the bright spot will appear on the side of the bubble away 

 from the mirror. This is an important experiment to make 

 whenever in doubt in this connection, as precisely the opposite 

 effect obtains in the oil drop, the bright spot, when oblique light 

 is used, then being on the same side with the mirror. Oil drops 

 may be prepared for examination by beating together a drop of 

 mucilage and one of clove oil. 



The beginner may have more trouble in experimenting with 

 oblique light than with central. About all that I can tell him is 

 to swing the mirror to one side, usually toward the right hand 

 as being more convenient, and then to manipulate it until the 

 light is properly reflected on the object, the degree of obliquity 

 of course varying with the position of mirror-bar, the mirror or 

 both. Frey in his work entitled " The Microscope and Micro- 

 scopical Technology," says that " Considerable practice is requi- 

 site with oblique illumination. The aperture of the stage must 

 be freed from diaphragms, or any other apparatus that may be 

 under the stage, and the various positions of the mirror are to^ 

 be tried while the eye is looking into the microscope. Truly 

 diabolical illumination is thus sometimes obtained, which, how- 

 ever, shows many fine details in an astonishing manner." Ob- 

 lique light as previously remarked, is chiefly used in the resolu- 

 tion of the fine lines on the surfaces of Diatoms, these little 

 ridges not being too minute to cast a shadow on the side oppo- 

 site to that from which the light is received. Oblique light is 

 occasionally used to produce delicate shadows when the micros- 



