MICROSCOPIC A^ISION. 147 



or balsam, and were called balsam angles. The word 

 "Homogeneous" as applied to immersion objectives is prob- 

 ably due to Tolles/ for he says, " Fig. 1 represents a 

 section of two hemispherical lenses balsam cemented, with 

 a diatom or other small object at the centre, together consti- 

 tuting a nearly homogeneous transparent globe." 



We must now leave the aperture question, and see what 

 was the state of the diffraction theory at that period. The 

 originator of the diffraction theories, both telescopic and 

 microscopic, was Frauenhofer, who, in 1821, ruled fine grat- 

 ings and measured the angular deviation of the diffracted 

 beams with a twelve-inch theodolite. From the measurements 



thus obtained he deduced the now well-known laws in ^ = ^ 







when 6 is the angular divergence of the diffracted beam, 

 \ the wave length, and 8 the value of a line and interspace. 

 From this equation he inferred that the limit of microscopic 

 vision was reached when 8 was equal to k ; in other words, 

 when the line and interspace equalled the wave length sin 

 became unity and 6 equal to 90°. 



Of course Frauenhofer knew nothing about oblique light 

 or immersion objectives, so that statement was substantially 

 accurate in his day. If you try the experiment, you will 

 find that you will be unable to resolve 48,000 lines per inch 

 with a wide-angled dry objective, when using a perfectly 

 parallel axial beam. Sir John Herschel, in 1827, said that 

 he was doubtful about the accuracy of Frauenhofer's con- 

 clusions. Nobert, the celebrated test-plate ruler, took up this 

 question in 1852 ; he, however, does not seem sure of his 

 ground, for he says, " I am therefore very anxious to learn 

 whether in resolving the lines of the test plate we shall be 

 able to progress beyond the 15th band." In the M. M. J. 

 1 ill, M, /., vol. vi. p. 214 (1871). 



