352 



KXO\vi,i:i)r,r.. 



Septemiikr, 1912. 



of his addresses as president of the Quckett Club, structure when one is said to be superposed upon 



when si)eakin{< of screens used in connection with another; Mr. F. J. W. Plaskitt, to wit, who speaks of 



objectives, says : — " They are of special service with the fibrils in Plctirosifima as beinf^ kaleidoscopic. One 



cheap lenses (i.e.. scmi-apochromats), because they wishes he had },'iven the chance to reply to his 



remove the secondary spectrum, making the lens strictures; jet, like the small boy in Punch, who 



as efficient as an apochromat. So remarkably is chalked up "No Popery," and then ran away, he 



Fk.iki. Jmo. T. fiiviis. same valve, to illustrate the 



foiiiiing of pseudi)he.\agons, black dot. Magnified two 



thousand five hundred diameters. 



this the case that 

 with equal apertures 

 it is impossible to 

 say whether the ob- 

 jective on the nose- 

 piece is an expensive 

 apochromat or a 

 cheap semi-apochro- 

 mat. In photomicro- 

 gra|)hy not only arc 

 the [ireceding remarks 

 applicable, but also 

 colours difficult to 

 photograph are ren- 

 dered neutral." 



The photomicro- 

 graphs obtained w itli 

 the Swift oil im- 

 mersion to illustrate- 

 the present article, 

 were taken in con- 

 junction with ;i 

 yellow screen upon 

 colour correct plates : 

 those with the Baker 

 objective upon ordinary plates, some 

 without a screen and the others in 

 connection with a pot green screen. 

 cutting off the red. This screen un- 

 doubtedly increases the resolving 

 power of the lens, besides producing 

 a beautiful image. In photographing 

 with lamp-light, however, it has the 

 disadvantage that the exposure must 

 be prolonged for at least fifteen 

 times bejond that required without 

 a screen. The average exposures 

 with no screen, ranged from ten to 

 fifteen minutes, with a yellow screen, 

 from a quarter to half-an-hour, and 

 with the green screen, three hours. 

 Unfortunately this is not the worst : 

 the chances are that after the three 

 hours one will find the focus shifted, 

 and the image useless. W'ith a 

 camera upon a common kitchen 

 table, over which each end projects 

 at least a foot ; with the table upon a 

 floor with wooden joists ; when it is 

 sought to use a magnification of 2750 diameters the found the cause. 



Flc.rUK J'Jl. T. favtis, from tlie same spot, to 

 show the white dot. Magnification the same. 



finds it more i)rudent to retire 

 from the contest. Figure J.SJ 

 of this article, however, should 

 settle the point. In a broken 

 valve of Pleitrosigina fonnosiim 

 the fibrils are seen projecting 

 forward into space, the corres- 

 ponding part underneath being 

 wanting, or rather having sunk to 

 the bottom against the slip. The 

 fracture where it has broken off 

 is show n in Figure 384, taken at 

 a lower focus. 



In Figures 3 and 3.\ of the 

 first article, there is shown in one 

 the normal appearance of the 

 outer side of the valve and, in 

 the other, the second layer im- 

 mediately underneath. Figure 9 

 lit the same article shows the 

 fibrils causing the appearance 

 naked, as it were ; but the letter- 

 press assigns no reason beyond 

 saying that the valve had been 

 acted upon in some way. This 

 lack of information, 

 one thinks, can 

 now be supplied 

 in the Figures 385 

 and 386 of the 

 present article. Of 

 :rse, it is under- 

 1 that the des- 

 ' 1 1 1 it ion can only 

 apply to a vah'e 

 mounted dry : w hen 

 mounted in a med- 

 itmi all such fine dis- 

 tinctions are obliter- 

 ated. In Figure 385 

 the fibrils are seen 

 placed lengthways 

 upon the valve, but 

 at intervals there are 

 collections of bright 

 discs of light en- 

 tangled amongst the 

 meshes. Underneath, 

 in Figure 386, is 

 The under surface is occu- 



conditions are not conducive to too much stability, pied b\- an ajiparent sieve-like structun- ; but 



The first part of the present article, now that we instead of being regular in size, many are 



are come to the subject, is rather an attempt at much larger, and in the grouping correspond 



meeting objections, of further amplications of the to the focal points of light in Figure 385. The 



last, than of raising new points. As to the first there is explanation seems to be this. Only the larger 



always a doubt in many minds as to the reality of apertures underneath ajipear to be capable of acting 



