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furnished with elevated ridges or lines running from one end to the 

 other ; and that the appearance of curved transverse lines, as seen by 

 ordinary light, is caused by crenulations in the upper edge of these 

 ridges. I might cite other cases, but these are enough to prove that 

 this arrangement is capable of showing forms of structure which 

 had not been previously recognised. 



" I ought here to say I have since found that Nachet's oblique 

 prism is capable of showing these peculiarities of structure, but I 

 have been unable to recognise their existence by any of the other 

 arrangements to which I have referred. 



" I was able to exhibit these effects to our President on the even- 

 ing of our first meeting this session, and soon after to other bro- 

 thers of the microscope, including my friend Mr. Grubb. I men- 

 tion this because I feel satisfaction in believing, as I do, that seeing 

 this was not without its effect in leading to the exceedingly beauti- 

 ful and efficient construction which he exhibited and explained to 

 the Academy a short time since. 



" I was unfortunately not here on that evening, but I have had 

 various opportunities of using his contrivance, and gladly bear my 

 testimony to the admirable arrangements he has effected for extend- 

 ing to the utmost limits the power of altering both the obliquity and 

 azimuth of the light while in the act of observing, with the further 

 advantage of being able, by means of graduated circles, to record 

 with facility and precision all the adjustments by which any phe- 

 nomenon has been observed. 



"In conclusion, I wish it to be understood that the present com- 

 munication is intended only to set forth the principles on which 

 depend the proper illumination of objects under examination by the 

 microscope ; and that where I have described apparatus, I by no 

 means consider it as perfect or as the best for the purpose, but I 

 thought it right to state the means by which I tested the views put 

 forward, hoping that others having more leisure and better qua- 

 lified than I am will turn their attention to the subject, and give to 

 the microscopist the best means of illuminating the objects of his 

 study, thereby enabling him to reap the full benefit of the improve- 

 ments which have been effected on the visual part of the instrument." 



