On the Fodura Scale. By E. M. Nelson. 399 



dark ground. Now when this illumination is in a direction from 

 the root to the tip a most striking image is obtained ; the tops of 

 the exclamation marks shine out like bright stars on the dark 

 ground of the scale. If the objective is a good one, and in correct 

 adjustment, these stars shine out clear and sharp, without any 

 flare or haziness of any kind. They are precisely like the electric 

 lamps which spell out the advertisements from the tops of tall 

 buildings in London. When the stage is rotated so that the light 

 falls on the scale in the reverse direction, viz. from tip to root, the 

 image is entirely changed, the electric lamps now shine with a dim 

 light, just as if sometliing had gone wrong, either with the current 

 or with the switch. Mr. Beck's figure shows a little speck of light 

 at the point of the exclamation mark, but actually it is in the same 

 place as it was before, viz. at the head. When the scale is illu- 

 minated at right angles to its length it appears very bright, the 

 whole of the exclamation mark being illuminated. It should 

 be noted, diowever, that this illumination, by means of an oil- 

 immersion condenser, is very much more powerful than any method 

 known in Beck's time ; consequently, the phenomena are more 

 easily observed.* 



Those wishing to pursue this subject, should read the excellent 

 papers by Messrs. Beck, Mclntire, Smith, and Wenham : the 

 references to these will be found in the attached bibliography. 

 The postage-stamp fracture appearance, which is obtained with the 

 vertical illuminator, is probably due to the oil or moisture running 

 in between the tops of the exclamation marks and the lower side 

 of the cover-glass ; we must always remember in interpreting this 

 kind of image that we are not dealing with a fracture, but with 

 structure which, though present, is obliterated by moisture. 



An examination of a Podura scale in a critical manner with 

 modern apochromatic objectives, illuminated by a full axial cone 

 from an achromatic oil-immersion condenser, shows that the 

 exclamation marks have V-shaped heads, causing them to assume 

 the appearance of the cuneiform marks on the Assyrian inscriptions. 

 Both the pin and knobbed heads of the exclamation marks are due 

 to some obliquity in the illumination, especially in a direction from 

 root to tip. When the full axial cone is perfectly centred the 

 knobbed and pin-shaped heads disappear. The tail of the exclama- 



* While working at a Podura scale, with this kind of illumination, two of these 

 minute electric lamps were observed close together, shining like a double star 

 when neatly split in a telescope. Objectives of less N.A. were then tried in suc- 

 cession, until one was found that just failed to divide the lamps. This objective 

 had a N.A. of 0-475. The last objective to just split the double had a N.A. of 

 0'5. An oil-immersion Jj was then placed on the nose-piece, and the distance, 

 from centre to centre, between these two particular exclamation marks, which 

 were leaning towards one another, was measured by a cobweb micrometer, and 

 found to be ^^^j^^ in. In the Table of Resolving Limits (this Journal, 1906, 

 p. 529) the limit for N.A. 0-475 is 33,100, and for N.A. 0-5 it is 34,800. So this 

 experiment and the tabular values agree. 



