On the Podura Scale. Btj E. M. Nelson. 401 



elevations which adhere closely to the scale for their entire 

 length. Occasionally, at a fracture, one is torn off from one side 

 and left sticking out beyond the edge of the other side, but such a 

 case is by no means common (see photographs, Dr. Mercer's figs. 5, 

 6 and 7, and Letherby's fig. 7). In Mercer's there are 72, and in 

 Letherby's 44 exclamation marks, either torn asunder, or bent 

 round an edge, and yet not a single spine protrudes clear from the 

 edge ; if they were either spines or featherlets it would be difficult 

 to find one not protruding over the edge. 



The experiment with polarised light seems to prove that the 

 structure, whatever it is, is considerably thicker at the exclamation 

 marks. Too much stress must not be put on Mr. Smith's con- 

 tention that the White Dot picture is to be regarded as more 

 correct than the Black Dot image. Black and White Dot images 

 are always complementary, and the Black Dot is not necessarily a 

 small cone image ; but Mr. Smith is quite right about the old- 

 fashioned black dot image, which was produced by a small cone. 

 An excellent photograph by Mr. O'Donohoe is seen on pi. 7, 

 p. 156, of this Journal, 1906. The | cone used is rather small, 

 nevertheless some of the surface markings between the exclama- 

 tion marks are visible. 



[The question as to whether the Black or White Dot is to be 

 accepted as the correct Microscope image is not yet settled, neither 

 has any satisfactory explanation of the phenomenon been pub- 

 lished. A somewhat similar image can be made with a telescope, 

 and at a focus on either side of the star-disk-and-ring image there 

 is an intensely black interference dot. This can be well studied 

 with sunlight and an artificial star. The distance between the two 



black dots, i.e. the one within and that without the focus, is ^^- , 



where X is the wave-length, p the conjugate focal length, and a 

 the diameter of the object-glass. Of course, when the object is 

 distant^ becomes /the principal focal length.] 



A large test scale from a Podura measures in inches yso loi^o 

 and 3^Q broad, and a large exclamation mark upon it measures 

 4 9^)0 lo^o' ^^^ ssioo broad, so roughly speaking, the exclamation 

 marks are just about one wave-length broad, and theii* length is 

 ten times their breadth. The true measurement of this exclamation 

 mark, correcting for antipoint, is 47V0 long, and 3-9500 broad. 



It is common knowledge that the Podura scale has played an 

 important part in the perfecting of early achromatic objectives. 

 These objectives are still extant, and for good correction and 

 definition they are even now hard to beat. They were driven out 

 of the market, not on account of their inferior quality, but because 

 of their high price ; for example, an old 90° | cost five guineas : 

 now a semi-apochromatic one can be purchased for twenty-five 

 shillings. 



