WITH THE MICROSCOPE. 2 39 



Dr. Woodward, besides these explicit details, iia the same article 

 has laid down the following principles : 



" i. To use objectives so corrected as to bring the actinic rays to 

 a focus. 



2. To illuminate by direct sun light passed through a. solution of 

 ammonio-sulphate of copper, which excludes practically all but the 

 actinic extremity of the spectrum. 



3. Where it is desired to increase the power of any objective, to 

 use a properly constructed achromatic concave instead of an eye- 

 piece. 



4. To focus on plate glass with a focussing glass instead of on 

 ground glass. 



5. With high powers to use a heliostat to preserve steady illumi- 

 nation. 



6. Where an object exhibits interference phenomena when illumi- 

 nated with parallel rays, as is the case with certain diatoms and many 

 of the soft tissues, to produce a proper diffusion of the rays by inter- 

 position of one or more plates of ground glass in the illuminating 

 pencil." 



Dr. Woodward sent me photographs of a part of a frustule of 

 Pleurosigma angulatum taken by him. The original negatives were 

 obtained in the one case by Messrs. Powell and Lealand's j^th, and 

 magnified to 2,344 diameters ; in the other case, by a -i-th, made by 

 Mr. Wales, of Fort Lee, New Jersey, and used with his achromatic 

 concave magnified to 2,540 diameters. Both of these negatives were 

 afterwards employed to procure positives, and from these, by one en- 

 largement, the enormous magnitude of 19,050 diameters. The former 

 gave, if anything, rather the sharpest picture, especially in the centre, 

 the latter the flattest field with most excellent definition. The 

 slight advantage in sharpness, attributed to the -g-Vth, Dr. Woodward 

 considers to be due to the chemical process employed. These nega- 

 tives were taken on collodion prepared plates, and the exposure 

 given was seven minutes. They certainly speak directly to the excel- 

 lence of the plan adopted, and the skill and patience of the operators, 

 Drs. Woodward and Curtis.* When we consider the number of reflect- 

 ing and absorbing surfaces and materials we may be prepared for 

 his lengthened exposure. It remains then to be tested whether by 

 adopting other plans we may not get rid of some of the expensive 

 parts of the apparatus, namely, the heliostat, and Dr. Maddox is now 



* Dr. Woodward in a letter to Dr. Maddox, March 25th, says, "a still more 

 perfect instrument is being constructed under the superintendence of Dr. 

 Curtis," and in a letter dated June igth, testifies to its great superiority and 



