20 Transactions of the Society. 



IV. — Li'/kt Filters for Photomicrography. 

 By E. Moffat. 



(Bead November 20, 1907.) 



Plate I. (figs. 3-6). 



For a number of years my attention has been directed to light 

 filters in connection with photomicrography, as a means of obtain- 

 ing well contrasted photographs of objects whose natural or faintly 

 stained appearance has occasioned one the greatest amount of 

 trouble, and in many cases had to be abandoned in despair. 

 Some fine pathological preparation, highly valued it may be, and 

 from which the stain has all but disappeared — to unmount and 

 re-stain which might be attended with considerable risk, owing to 

 the fineness of the texture — or some very pale-yellow insect dissec- 

 tion, or other difficult object, has to be photographed : without a 

 filter and orthochromatic plate the attempt would be well nigh 

 hopeless, but given a correctly prepared filter to meet the special 

 needs of the case, the result will be highly gratifying. 



The spectroscope in such cases is invaluable in the determina- 

 tion of colour and depth of tint required. A simple pocket instru- 

 ment is sufficiently good, but where greater accuracy is demanded, 

 one can easily rig up a table instrument with two cheap tele- 

 scopes — one being used as a collimator, using the object-glass 

 only, and the other is easily converted into a small astronomical 

 telescope ; a dense glass prism and slit completing the arrange- 

 ment. A scale can be made on paper of the principal Fraunhofer 

 lines, and this can be used for recording by artificial light, where 

 the absorption bands appear on the spectrum of the dyes or filters 

 we are about to employ. 



If we place in a cell of about 1 cm. deep a weak solution of the 

 dye by which the preparation is stained, we shall find that the 

 spectrum is modified, and some parts may be missing altogether, 

 as where the absorption bands appear. Now, to obtain the greatest 

 sontrast, we must photograph in the absorption band region with a 

 filter which will always be the complementary colour, and therefore 

 produce the greatest darkness upon the resulting print, the shadows 

 upon the negative having practically clear glass ; e.g. fuchsin gives 

 a band about midway between D and E, and is well met by a screen 

 or liquid filter composed of a saturated solution of copper acetate 

 and a little potassium bichromate. These may be made up in two 



