THE VICTORIAN NATURALIST. 



163 



NOTE ON MICROSCOPICAL TEST RULINGS. 

 Natural tests, such as diatom frustules, have long been used 

 for testing the resolving and defining power of microscope lenses. 

 They are, however, very variable as regards the " fineness " of 

 the striations and other markings in different individuals of the 

 same species, and their employment as tests is, therefore, open 

 to doubt. To supply the want of a recognized standard, the late 

 Herr Nobert conceived the idea of producing artificial tests in 

 the form of ruled bands of lines of increasing fineness and close- 

 ness. His rulings have contributed very largely to improvements 

 in the manufacture of microscopic lenses, and have never been 

 excelled. 



More than a year ago it occurred to me that I could produce 

 a series of lines on the pattern laid down by Nobert, and thus 

 bring these tests within the reach of our local microscopists. 

 The test rulings exhibited this evening are the outcome of my 

 experiments in this direction, and I venture to state that they 

 will bear the strictest investigation. In the construction of the 

 machine with which the lines are produced I have had the 

 assistance and advice of Mr. W. Stone. The conception and 

 initiatory steps of the work and the ruling of the lines have fallen 

 to me. 



As regards the details of rulings on the slides exhibited, the 

 first slide shows n bands of lines in regular series. The first 

 band in the series includes lines drawn at the rate of 5,000 to the 

 inch, and successive bands (the last excepted) increase by 5,000, 

 the last one registering 60,000 lines to the inch. On the second 

 slide the bands range from 5,000 to 100,000 in increasing in- 

 tervals of 10,000 lines per inch, whilst on a third slide the ruling 

 is continued in the same manner up to 120,000, without showing 

 any signs of breaking down. The resolution of a band of 120,000 

 lines to the inch is said to be the limit of optical resolution with 

 the most perfect lens and illuminating apparatus. One of the 

 bands referred to is shown clearly resolved up to 80,000 under 

 a T V oil immersion. With attention to the details of illumination, 

 lines in the 100,000 band can just be discovered. That the lines 

 in the higher bands exist is shown by the fact that they exhibit a 

 spectrum when properly illuminated for the purpose, no rupture 

 in the band being apparent. 



In addition to the test rulings several micrometers are shown 

 in the spacing of which it is impossible to detect any errors. A 

 standard scale for these rulings has been obtained from the Mel- 

 bourne Observatory. A mean reading of 20 centimetres is taken 

 as a basis for a miscroscopic standard of one centimetre, which 

 may be regarded as accurate to within the 50,000th of an inch. 

 The English inch values are calculated from this, and further 

 compared with an accurate copy of a Troughton and Sims's scale 

 (also from the Observatory). Fifteen millimetre rulings, super- 



