579 



X. — A Micrometric Correction for Minute Objects. 

 By Edward M. Nelson. 



{Read June 17th, 1903.) 



Most microscopists are aware that the magnified image of an 

 object is made up of diffraction discs, to which Mr. Gordon has 

 given the appropriate name of " antipoints "; they will agree there- 

 fore that some notice must be taken of the size of these discs when 

 the micrometry of minute objects is performed. 



A reference to fig. 139 will make this clear. A and B are the 

 webs of a micrometer. The large circle represents the true magni- 

 fied image of the object : this is never seen ; the small circles 



A 



C 



LI 



CD (T 



D 



Fig. 139. 



Fig. 140. 



are the antipoints. It is obvious that if the true size CD of the 

 magnified image is required it will be necessary to subtract the 

 diameter of one antipoint from the measure given by AB. In this 

 case the object is illuminated on a dark ground. 



Fig. 140 shows the same magnified image of the object illumi- 

 nated on a bright ground ; here the antipoint eats into the edge of 

 the image and the measurement A B must be augmented by the 

 diameter of one antipoint before the true size of the image can be 

 known. 



The only datum required is the- size of the antipoint. A glance 

 at figs. 139 and 140 shows that it is half the difference of the two 

 readings of the webs A B. 



The effect of the antipoint on the magnified image can be very 

 easily demonstrated ; take, for example, the proboscis of the blowfly, 

 and examine with a ^-in. objective the very minute hairs on, or 

 protruding beyond the edge of the delicate membrane ; illuminate 

 them first on a bright field by a f cone ; now notice the almost 



