284 THE USE OF THE MICROSCOPE 



the object, with too large a source-field; and also from reflections in the 

 cover-glass, or slide, or mounting medium. (Beck.) 



Glass, Jena. Borates, phosphates and other new materials were 

 used in a series of glasses, tested for their R. I. and dispersion, as well 

 as for durability. 



Homal. A concave combination used as an amplifier, with an objec- 

 tive, to project an image for photography at a fixed distance. It has the 

 same corrections as the compensating eyepieces; and in addition, 

 flattens the field. Different homals are made for the low dry, the high 

 dry, and the immersion apochromatics. (Boegehold.) 



Huyghenian eyepiece. Eyepiece with the image between the simple 

 lenses, one of which is a field lens. Negative eyepiece. 



Hypermetropy. State of the eye when it cannot focus, even for 

 distance, without the assistance of a convex lens. 



Illumination of the object is determined (1) by the intensity and (2) 

 by the aperture of the light falling on each point. 



Image, real. Place where the rays cross, or where the wave fronts 

 converge to more or less expanded points (aberration discs). 



Image, virtual. Place where the rays would cross if prolonged 

 backwards. 



Image field. The circle of the image seen through the eyepiece, 

 calculated as if at 250 millimeters distance. Usually about 6 inches 

 across. 



Imbedding. Saturating more or less permeable material with a 

 melted substance in lieu of its natural more or less aqueous contents. 

 Usually applied to the graded replacement of water by melted paraffin 

 in plant or animal tissues. 



Immersion. Putting a layer of liquid between an objective and the 

 cover-glass, or between the slide and the condenser. The liquid most 

 used for immersion is thickened cedar oil. Specially made objectives 

 and specially corrected condensers are used as water immersion, which 

 is a convenient form, and gives optical results only a little inferior to 

 oil-immersion, though requiring more skill. The inferiority is partly 

 due to the lower aperture of the objectives, 1.25 as compared with 1.4. 

 But on objects in water, the water hnmersion is often to be preferred; 

 for here the aperture of the oil objective is reduced to or below a limit 

 of 1.33. Glycerin immersion is used with quartz objectives for ultra- 

 violet light. 



Incidence, angle of. The angle between a ray and the perpendicular 

 to the surface. 



Index of refraction (R.I.). The ratio of the speeds of light in vacuo 

 and in the substance in question. Measured by the ratio of the sines 

 of the angles of incidence and refraction. 



Intensity of light. A property of the source. No optical api)aratus 

 can increase the intiMisity of the source in an image, but only shifts the 



