The Microscope, 145 



device like the little hemispherical lens called by Mr Tolles his 

 " Traverse Lens," which being attached to the under side of the 

 slip by an immersion contact, allows the rays to pass into the 

 slide without refraction, and consequently gives the angle in 

 terms of the glass or the homogeneous immersion angle. When 

 this device is used with a dry objective or a water immersion, 

 it becomes necessary to translate the aperture indicated by the 

 movement of the source of illumination, which is the angle of 

 homogeneous immersion, into the corresponding angle for air or 

 water as the case may be. This is easily done by a very simple 

 calculation founded on the law of sines and the rule of three, and 

 still more easily by reference to the table of aperture published 

 by the Royal Microscopical Society as part of the cover of each 

 number of their Journal, and republished in this country in 

 various places, of which the catalogue of the Bausch and Lomb 

 Optical Company, of Rochester, N. Y., and the Proceedings of 

 the American Society of Microscopists for 1883, are perhaps the 

 most readily available. 



The plan of measurement here outlined is sufficiently accurate 

 for such approximate determinations as are within the technical 

 skill of the non-expert for whose benefit the working sessions^ 

 demonstrations are especially intended. 



If the microscope stand is provided, as it should be, with a 

 radial arm for the mirror, and a means of reading the obliquity 

 to which it is swung, it is easy to substitute a toy candle for the 

 mirror and to proceed as directed in the foregoing. If the stand 

 is not provided with a radial arm, the process becomes a little 

 more complicated, but there are numerous methods for deter- 

 mining the angular value of the distance through which the 

 source of the illumination is moved, which will readily suggest 

 themselves to any one with even a slight acquaintance with 

 mathematics and trigonometry. 



If greater accuracy is desired, it can be secured by the use of 

 an opaque slide with a transparent line across it in which the 

 objects are mounted, and this line can be so placed on the stage 

 as to bisect the field of view very accurately in a vertical direc- 

 tion, and the exact moment at which the centre of the field is 

 darkened can thus be determined with greater precision. Such 



1, This paper was read before the Buffalo meeting of the American Society of 

 Microscopists, at the working session. 



