240 HOW TO WORK 



experimenting in this direction, by means of a solar microscope. He 

 finds on an ordinary collodion sensitised plate, bath in good condi- 

 tion and iron developer, the exposure with a |th, and achromatic 

 concave, a large plane silvered mirror, a 3 ^-inch diameter and 

 8 j-inch focus condenser, and a single pair of plano-convex con- 

 densers with a large central stop, the time necessaiy for a negative of 

 Pleurosigma angulatum magnified 2,500 diameters was under strong 

 sun light in December 90 to no seconds, in May 70 seconds for 

 Pleurosigma formosum. He attributes much of the lengthened expo- 

 sure to this |th being made with 4 sets of lenses, the front a single 

 lens. With an excellent -gth with three sets of lenses and an achro- 

 matic concave, made for him by Mr. W. Wales, of Fort Lee, New 

 Jersey, U. S., especially for photographic purposes, the time of expo- 

 sure is rather less ; and with a triple condenser 35 seconds in June, 

 for Pleurosigma angulatum magnified 3,000 diameters using the am- 

 monio-sulphate of copper cell. From a short experience with this 

 instrument, both with and without the ammonio-sulphate of copper 

 cell, he thinks a prism either after the plan used in his smaller 

 camera arrangement or as adopted by M. Neyt and Count Castracane 

 preferable to a mirror for illumination with the high powers. To 

 adapt this objective to ordinary use, Mr. Wales supplies a separate 

 back set of lenses to replace the photographic set, which answers 

 well, the workmanship in the construction of the mount being most 

 perfect. 



There is every reason to suppose the very valuable invention of 

 Mr. Wilde, in magneto-electricity might be most successfully em- 

 ployed for this purpose ; perhaps the lamp invented by Mr. Larkin, 

 for consuming powdered metallic magnesium mingled with sand and 

 allowed to fall in a stream through a small lighted jet of hydrogen 

 gas. Whichever plan may be selected, the chief object is to have the 

 intense light necessary, emanating from a small surface, so that it can 

 be more successfully brought to a focus by a condensing lens or sil- 

 vered reflector, singly or united. At the end of this part attention 

 is directed to the use of magnesium wire, &c. The Rev. St. Vincent 



efficiency over the arrangement described. Since the above was written, Dr. Mad- 

 dox has received from Lieutenant-Colonel Dr. Woodward for publication, an 

 important illustrated paper, in which he gives the results of some experiments 

 made to test the value of the actinic rays emanating from a flint prism at the violet 

 end of the spectrum, as compared with the illumination from a silvered mirror, 

 the rays being transmitted through a cell containing a solution of the ammonio 

 sulphate of copper. Considerable advantage is shown in favour of the latter plan. 

 The test, however, does not decide the question as to the advantages of a con- 

 densing prism, or an ordinary silvered mirror, but Dr. Maddox has always con- 

 sidered that the former affords the best results. See Appendix to this work. 



