1G6 J. DEBY "KECEIPTS FOR MICROSCOnSTS." 



often great, even when using the admh-able micrometers invented by 

 ]\Ir. Browning and Mr. Sorby. I find that in most practical 

 cases the microrspectrum can be thrown upon a sheet of white 

 paper by means of an ordinary camera lucida placed over the eye- 

 piece of the spectroscope. Strong light by means of a condenser 

 has to be thrown through the liquid under examination. By means 

 of an ivory rule, finely divided, and brought back to a known line, 

 say D, all other lines or bands may be directly measured off on the 

 rule, and, if desired, the exact results in raillionths of a millimetre 

 may then be computed by any of the known interpolation formulae, 

 such as are given in Suffolk's useful little book.* 



III. — The arrangement of small microscopic objects, such as 

 diatoms, foraminifer^e, etc., on slides in regular lines, circles, or 

 patterns, can be much facilitated in the following way : — Draw 

 with a pen and ink cross lines, or circles, or any other figure required 

 on the surface of the plain mirror of the microscope ; then focus 

 down until the image of these lines is seen on the upper surface of 

 the top lens of the condenser. By means of a mechanical finger, or 

 of a steady hand with a rest, no difficulty will now be experienced 

 in placing the objects in perfectly regular order. 



IV. — I now obtain excellent condensed monochromatic light by 

 means of a bull's-eye of unusual external shape, the internal 

 portion of which, however, is filled with glycerine or oil of cloves 

 coloured to suit. This bull's-eye has a plane back and a concavo- 

 convex front, and the liquid is introduced through a hole in the flat 

 side, closed by a small ground stopper. This apparatus is furnished 

 with universal motions, and has a rack and pinion foot. It was 

 made for me by Mr. J. Browning. When using blue light, produced 

 by ammonia sulphate solutions, I have resolved, by means of this 

 monochromatic bull's-eye amphipleura, with objectives in my pos- 

 session, which will hardly show Pleurosigma angulatum under 

 ordinary condenser illumination. 



V. — Some time ago, Mr. J. E. Ingpen, on my behalf, made a 

 communication to the Club in regard to a growing slide I had devised 

 for some special researches I was following at the time. Some diffi- 

 culty seems to have been found in the making of these slides, so 

 that it is with pleasure I now offer a still more simple contrivance 

 for obtaining the same results. Here is the receipt : — Take an 



* " Spectrum Analysis as Applied to Microscopical Observation." W. T. 

 Suffolk, F.ll.M.S. Loudon, John Browning, 63, Strand. 1873. 



