224 PKACTICAL PHOTO-MICBOGRAPHY 



actual working N.A., and with this object in view a complete 

 table, basing exposures solely on this factor, was laid before 

 the Koyal Microscopical Society on October 16, 1907, by 

 Mr. Alfred Latherby. A full report of his paper is to be found 

 in the Journal for that year. While this method would appear 

 to work very well with a range of objects that did not present 

 any considerable colour or contrast difficulty, it would be of 

 little assistance in cases where widely varying objects had 

 to be dealt with. The difficulties of exposure are most in 

 evidence, of course, to the beginner; to such the method 

 may be commended as enabling him in the course of his 

 work to obtain a series of records that would form a basis 

 for approximate estimations. The method works out in 

 practice exactly on the lines for determining the illumination 

 of any area by an illuminant at varying distances that is, that 

 the intensity of illumination varies as the square of the distance. 

 Hence with a N.A. say of 1 and a magnification of 100, an 

 object would require one-fourth the exposure that it would with 

 a magnification of 200, and one-sixteenth with a magnification 

 of 400, assuming that all other factors remain the same. It is 

 not a difficult matter, where the working N.A. is known, to 

 formulate a table from a few trial exposures which will suffi- 

 ciently indicate what is needed with varying magnifications. 



The weak point about the method is that it is not always 

 easy to estimate exactly the working N.A. To remedy this 

 and supplement the method, Dr. Duncan J. Eeid has suggested 

 that this factor may be determined by measuring the diameter 

 of the Kamsden disc above the ocular. The Ramsden disc is 

 that point above the ocular where the rays of light after emerg- 

 ing from the eye-piece converge to a point ; and the diameter of 

 this circle varies with the aperture of the objective and the 

 power of the ocular. The greater the magnifying power of 

 the latter, the smaller does this disc become. If this small 

 disc is projected on to a transparent millimetre measure, 

 and by means of an ordinary hand-magnifier its size is read off 

 say in millimetres and tenths it is possible to determine its 

 diameter directly. Half this diameter should be multiplied by 

 the magnifying power of the ocular (which is usually known), 

 and the total divided by the focal length of the objective; the 

 result gives the N.A. of the objective in use. As an example, 



