MICROSCOPIC ILLUMINATION. 85 



(4) When the iris of the illuminant is fully open, a large 

 uniform source is obtained suitable for low-powerwork, by closing 

 down the iris, or by slipping in small perforated discs the illumina- 

 tion may be restricted to a portion of the field, even with a 

 magnification of 1,000 diameters. 



(5) The intensity of the illuminant may be controlled at will 

 by means of a simple adjustable resistance. 



(6) The illuminant is not visible to the observer. 



(7) Optical errors are not introduced by the use of the plane 

 mirror. 



The first three advantages allow the microscope, Avhen removed 

 from its case, to be ready in complete adjustment for immediate 

 use. Further, very seldom is it necessary, since the condenser 

 is an oil-immersion system, to make changes in the positions of 

 any of the parts of the substage system. (Flint slides, or those 

 in which realgar is used as medium, are the only cases requiring 

 readjustment that I have found.) The disadvantages of using 

 an oil-immersion condenser are entirely due to slides not being 

 of a standard thickness, since some slides are too thick for the 

 working distance of the condenser, while others are so thin that 

 the immersion fluid tends to leave the gap between the slide and 

 the condenser front. Water has therefore been proposed as an 

 immersion fluid, since its high-surface tension tends to hold it in 

 place by capillarity. Its refractive index is, however, so low that 

 different thicknesses of slide require different distances between 

 the condenser and illuminant. Ainslie (1) has suggested the use 

 of glycerine, but its refractive index is still found to be too low 

 for its employment as an immersion liquid. The use of the 

 immersion condenser in practice would, hovvcver, appear to be 

 facilitated in the following ways : 



(1) To provide a simple gauge which would test whether a 

 slide is too thick, correct, or requiring to be increased in thickness 

 by the addition of another layer of glass. 



(2) To employ an immersion fluid of the same refractive index, 

 but much higher surface tension than that of cedarwood oil. 



Both these suggestions have been tested, and the results will 

 be published in detail in a future paper. The arrangement of 

 condenser and substage described above in fig. 2 has been found 

 suitable for objectives from l/3rd in. to l/12th in. The diameter 

 of the illuminated portion of the slide was found by measure- 



