Object-glasses and their Definition. 21 



some other means than mere sight, and obtaining more mechanical 

 evidence of their form. I said that in a doubled- up scale " the 

 markings ply round the sharp bend so closely that the keenest eye 

 cannot detect any appreciable rib or projection ;" but this assertion 

 I must withdraw, as Mr. Beck informs me that in the coarse speci- 

 mens, of which he has a variety, the ribs can be seen plainly 

 projecting at the bend ; and his recent communication to the Eoyal 

 Microscopical Society concerning the moisture experiment, furnishes 

 another proof that the markings are longitudinal ribs projecting 

 mostly on one side of the scale, and are not caused by similar rows 

 of beads crossmg obliquely on opposite sides as Dr. Pigott states. 

 Whether these "beads" in the form stated are fallacious or not 

 must be left for the present; there is probably some analogy 

 between them and those on the ribs of butterfly scales; but if an 

 object-glass fails to discover any indications of them, the fault may 

 be in the scale — some of which will show nothing of the kind, for 

 there is a singular difficulty in obtaining boldly-marked scales in 

 some localities. Dr. Pigott says that in my last communication I 

 have " sketched the Podura markings something like an Irishman's 

 shillalah." Not being familiar with the look of that atrocious 

 weapon, I do not feel the force of the simile ; but I have a very 

 beautiful and exquisitely distinct photograph of the Podura, kindly 

 sent to me by Colonel Dr. Woodward, one edge of which has roimded 

 away from the cover to which it adheres, bringing the markings 

 gradually into profile in the form of my sketch. But Dr. Pigott 

 repudiates photography, saying that it fails to reveal what the eye 

 alone can detect, and that the photographer has never yet displayed 

 the AmjjJiipIeura pellueida — simply, I presume, because it has 

 never yet been tried. When markings have been very difficult to 

 show by ordinary illumination, I have frequently been surprised at 

 the wonderful facility with which they are displayed in a solar 

 image ; in fact, Dr. Woodward's photographs of Nobert's 19th band 

 magnified 2800 diameters (a series of which he has also sent to me) 

 tend to prove this after the long discussion as to whether they have 

 been seen or not. 



I must protest against the positive manner in which Dr. Pigott 

 still totally condemns the mercury globule as a test for the con- 

 struction of object-glasses. This self-confidence may be attributed 

 to non-practical acquaintance with the use of it. That it is em- 

 ployed solely for the purpose of obtaining a distinct image is quite 

 a mistake. The greatest value of its indications are when it is 

 considerably within or without the focus. Commencing with the 

 back triple, this is first corrected alone on the globule, and the coma 

 observed both within and without the approximate focus, gives the 

 indication whether the lenses are right, or what alterations will be 

 needed to make them on. No optician thinks of the possibility of a 



