1893.] THE MICROSCOPE. 3 



Air Bubbles with Central Illumination. — Shade the object ; 

 and with the plane mirror, light the field with central light. 



Search the preparation until an air bubble is found appearing 

 about i mm. in diameter, get it into the centre of the field, and if 

 the light is central the air bubble will appear with a wide, dark, 

 circular margin and a small bright centre. If the bright spot is 

 not in the centre, adjust the mirror until it is. 



This is" one of the simplest and surest methods of telling when 

 the light is central or axial. 



Focus both up and down, noting that in focusing up the central 

 spot becomes very clear and the black ring very sharp. On 

 elevating the body still more the centre becomes dim, and the 

 whole bubble loses its sharpness of outline. 



Air Bubbles with Oblique Illumination. — Remove the 

 substage of the microscope, and all the diaphragms. Swing the 

 mirror so that the rays may be sent very obliquely upon the 

 object. The bright spot will appear no longer in the centre but 

 on the side away from the mirror. 



Oil Globules. — Prepare these by beating a small drop of 

 clove oil with mucilage on a slide and covering as directed for 

 air bubbles. 



Oil Globules with Central Illumination. — Use the same 

 diaphragm and light as above. Find an oil globule appearing 

 about i mm. in diameter. If the light is central the bright spot 

 will appear in the centre as with air. Focus up and down as 

 with air, and note that the bright centre of the oil globule is 

 clearest last in focusing up. 



Oil Globules with Oblique Illumination. — Remove the 

 substage, etc., as above, and swing the mirror to one side and 

 light with oblique light. The bright spot will be eccentric, and 

 will appear to be on the same side as the mirror. 



Oil and Air Together. — Make a preparation exactly as 

 described for air bubbles, and add at one edge a little of the mix- 

 ture of oil and mucilage ; cover and examine. 



The substage need not be used in this experiment. Search the 

 preparation until an air bubble and an oil globule, each about i 

 mm. in diameter, are found in the same field of view. Light 

 first with central light, and note that in focusing up the air bub- 

 ble comes into focus first and that the central spot is smaller than 

 that of the oil globule. Then, of course, the black ring will be 

 wider in the air bubble than in the oil globule. Make the light 

 oblique. The bright spot in the air bubble will move away from 

 the mirror, while that in the oil globule will move toward it. 



Air and Oil by Reflected Light. — Cover the diaphragm or 

 mirror so that no transmitted light can reach the preparation, 

 using the same preparation. The oil and air will appear like 

 globes of silver on a dark ground. The part that was darkest in 



