Dr. Col Woodward to the •' Colour Test." 65 



On Using the Colour Test to determine Focal Depth, and the 



Nature of certain Structures. 



The following observation illustrates the value and importance 

 of the colour test in examining minute structures. 



The object employed was a minute triangular fragment of 

 P. Formosum, mounted in Canada balsam, exhibiting nine beads 

 on one side and six on the other, the longer side being formed of a 

 portion of the mid-rib. The focus was gradually, but very deli- 

 cately deepened. Appearances — 



Fi. First focus. All the boundary beads appeared blue and in 

 close contact. An inner row in contact, but 

 pale and indistinct. 

 F2. Second Focus. Outlying beads reddish, in close contact. Inner 



row touching them, blue. 

 F3. Third Focus. Outer beads indistinct. Inner red. 

 F4. Fourth Focus. Boundary beads vanished. Inner beads dark red. 



The planes of these several foci corresponded to focal changes 

 of about 1-60, 000th of an inch ; so that the focus deepened each 

 time by about half the depth of the vertical diameter of a bead. 



Conclusions. 1. Never having had before the good fortune to 

 discriminate so clearly the double set of beading of the Formosum 

 formerly described by me, as in this broken fragment, I am now 

 enabled to declare, that this application of " the Colour Test " 

 determined the fact that the two sets of beading occupy different 

 planes, i. e. the outer boundary beads were elevated above the inner 

 rows. 



2. That instead of the Formosum being composed of beading 

 usually reckoned at about 33,000 to the inch, the beads are set 

 much more thickly (cannon ball- wise), one layer of beads being 

 piled upon the other layer, so as to admit the beads of the lower 

 layer to appear between those of the upper. By means of Brown- 

 ing's delicate Kecording Micrometer, which under the power used 

 gave 924 divisions for a thousandth of an inch on the stage, I 

 ascertained the diameter of the beads in contact was about the one 

 fifty thousandth of an inch. A good idea of the structure of the 

 Formosum would be obtained by examining a layer of shot loosely 

 spread out, but confined by a frame upon which another layer had 

 been superimposed. 



Having constructed a measuring machine which was so delicate 

 as to admit of changing the central spot of Newton's Eings through 

 the nine orders of colours, in which I counted altogether twenty- 

 three changes, under the microscope 1 hope to be able to survey 

 with great accuracy the position of the molecules of structure, 

 assisted by the colour test here described, and so determine more 

 accurately the depth of the different layers. 



