INFrSORT.VL ANIMALCrLES. 103 



neuti-alize the yellow. To obtain the correct tint of blue for this 

 pui'pose, it has been proposed to select a crystal, which, by polarized 

 light, gave the exact yellow tint of the candle, and then, by turning 

 the polarizer a quarter round, you have the crystal of the exact blue 

 required. 



This method of sifting the light, leaves for use only so much from 

 the candle as is imperfectly monochromatic, which, I am inclined to 

 think, is too feeble. Obscrvci-s should bear in mind the results of 

 the observations in the Microscopic Ilhistrations, on this subject, 

 namely, that a piu'c intense light is far more important than quantity. 

 In viewing live animalcides, it will be well to be provided with 

 diaphi'agms or stops to place imder yoiu" object. By the employ- 

 ment of these, you ^oll the more readily be able to detect cilia, alsa 

 their an'angement and the civrrents in the water produced by cilia. 

 In viewing the striae on the lorica of the BaciUaria, stops or diaphragms 

 must not be used. In general, oblique light is best to demonstrate 

 their existence. When Dr. Goring and myself were fii'st engaged 

 upon test objects, it was laid down by us as an axiom, that all lined 

 objects required oblique light. Subsequent improvements made in 

 consequence of our pointing out the value of angidar apertui'e- 

 (imknown until Dr. Goring discovered it) enabled microscopic ob- 

 servers to see the lined tests of those days with direct light. This- 

 induced some observers to deride the idea that oblique light was- 

 at all necessary. The recent discovery of more difficult tests, viz., 

 the minute shells of certain Kaviculfs, has shown that, even with our 

 most improved achromatic microscopes, it is necessary, nay, so 

 ticklish a thing is the illumination, that I am informed that those- 

 who make the structui'e of these shells a special study, require 40 or 

 50 minutes manipulation ■with a first rate instniment, and all the 

 modei'n ingenious appliances for obtaining intense oblique light, before 

 they can show certain stria; or dots on a well-known specimen. 



While treating of oblique illumination, I may mention that, if 

 artists can produce object-glasses of increased angular aperture, it is 

 probable we may commence another cycle in om* modes of observa- 

 tion, and then view all the present difficult tests with centi'al light. 

 Mr. Johnson, of New York, states that such has been done by Mr. 

 Spencer, who has produced an object glass of one-twelfth of an inch 

 focils and 147° of clear aperture. However, so long as oblique light 



