40 



compare the different families together, and how they differ 

 from other groups. 



The objects are attached to the covering glass, like Nobert's 

 test lines, which is very thin, so that it allows the use of high 

 powers ; both sets of lines on Navicula JRhomloides are 

 resolved with a one-twentieth objective, so that each Diatom 

 may be separately examined with deep lenses. The thickness 

 of covering- glass required for focusing under high powers 

 will explain the difficulty of using them. For a l-12th and 

 l-16th objective the covering-glass must not exceed '004 in. 

 "For l-20th, l-25th, and l-50th, -005 in. It was my intention 

 to have sho^vn Moller's Diatom Typen Plate, and Robert's 

 bands of Test Lines, under the microscope, at the annual 

 soiree which the Council have fixed for the 1 6th of November, 

 but from a correspondence by the last mail, I learn that they 

 are not to be had, except at a very great price. I have re- 

 ceived, however, from members of the Eoyal Miscroscopic 

 Society, some specimens of diamond writing, &c., equally 

 marvellous and interesting, and much better suited for the 

 purpose, which were engraved and exhibited at the Inter- 

 national Exhibition, class 13. The following is a specimen : 

 — " A micrometer with divisions of 1-lOOth, 1-1,000, l-2,000th, 

 1 -4,000th of an inch, with 40 parallelograms, each of 

 l-100,000th of an inch ; 40 parallelograms each of l-200,000th 

 of an inch ; 80 parallelograms each of l-400,000th of an inch ; 

 200 parallelograms each of l-2,000,000th of an inch ; 400 

 parallelograms each of l-4,000,000th of an inch ; and 400 

 parallelograms each of l-8,000,000th of an inch ; with squares 

 of l-10,000th of an inch ; 100 squares each of l-l,000,000th of 

 an inch ; 100 squares each of l-4,000,000th of an inch ; and 

 400 squares each of l-16,000,000th of an inch. The Lord's 

 Prayer, the Creed, God Save the Queen, Eule Britannia, a 

 geometric coil, the 4,137 letters in the 51 verses of the 1st 

 Chapter of St. John, a poem upon the International Exhibi- 

 tion, from copper plates engraved in the Exhibition, each in a 

 space from l-l,606th to l-2,000th of an inch. A Freemason's 

 sign, which can be covered by a pin's point, &c. 



There are various means employed for measuring these 

 delicate objects, the best is undoubtedly the position micro- 

 meter, the same as the one used with the telescope for 

 measuring the distance and angle of double-stars ; but this 

 instrument is both expensive and complicated. The method 

 now in general use is the micrometer eye-piece, this is an. 

 eye-piece either positive or negative, having a divided glass 

 micrometer fitted into a brass frame and placed in its proper 

 focus. If the positive eye-piece is used, the micrometer is 

 placed below the field-glass, but if the negative eye-glass is 



