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Mr. J. B. Haycraft. Upon the Cause of [Feb. 3, 



fibre will modify and change the direction of rays passing through it, 

 so will also a stained fibre produce what are apparently modifications 

 of the staining effect. It is generally stated that the dark band and 

 Dobie's line are stained by logwood and carmine, while the bright 

 bands remain unaffected ; also that Hensen's stripe in the centre of 

 the dark stripe is stained only to a slight degree : whence it follows 

 that if staining action is to be the criterion, this stripe differs in struc- 

 ture from the dark stripe. 



We, however, affirm that the whole fibre is stained, and equally 

 stained throughout. The bright band is undoubtedly stained, although 

 it appears not of the deep blue of the dark stripe when coloured by 

 logwood ; and this conclusion is drawn not only from an examination 

 of my own specimens, but also from some of great beauty shown to 

 me by Dr. Klein. Why the bright band does not appear of so dark a 

 blue is, that the apparent shading of the latter is added to the blue 

 tint, producing a depth of colour. The most conclusive proof of this 

 is, that one can often reverse the colouring on readjusting the focus, 

 and that Hensen's stripe or the bright part of the dark stripe is only 

 of a faint light-blue, like that of the bright stripe. 



Picric acid stains muscle very readily, and colours it throughout. 

 The fibre to the naked eye is yellow and uniformly so, but when 

 examined by the microscope, alternating yellow and shaded yellow 

 bands are to be observed, which reverse their position on changing 

 the focus. With a high focus — when the crests are bright in the un- 

 stained preparation — they are of a bright yellow, while the valleys are 

 of a deeper yellow tint. 



To show the effects which a fibre of this shape can produce when 

 transmitting monochromatic light, nothing can be more conclusive 

 than the following experiment. A slip of coloured blue glass is held 

 obliquely between the reflector and the stage of the microscope, so that 

 blue rays pass through the fibre. It does not appear of a uniform tint, 

 but beautiful blue stripes are seen corresponding with the crests and 

 valleys, and varying with alterations of focus. If a piece of red glass 

 be substituted for the blue slip, red cross stripes are seen in corre- 

 sponding places. For this experiment the fresh fibres of insects' 

 muscle should be examined, for, with fine mammalian muscle, the light 

 is not so good, owing to the higher power required. This experi- 

 ment has been introduced here with the description of stained muscle, 

 not that it can be strictly compared with an ordinary staining process, 

 but simply to show what an influence the fibre's shape must have upon 

 the tinting, supposing, as we do, that this is in reality uniform. 



An investigation such as this is beset with many difficulties and 

 fallacies, and I may mention one which bef el me in this stage of my 

 work. 



I had stained a few muscular fibres of a rabbit with picro- carmine, 



