Micro-ruling on Glass and Steel. 151 



piece, 900 diameters -when used wwcovered, and 1100 when used 

 covered ; and it is certainly highly creditable to an objective of so 

 much lower power to be capable of resolving so difficult a test as the 

 Amphipleura. 



From this performance I expected that the new ith would at 

 least resolve the sixteenth band of the Nobert's plate ; but in this I 

 have hitherto been unsuccessful, getting indeed handsomely through 

 the fifteenth band with it, but no fm'ther. Now, as the objective 

 resolved Amphipleura frustules with striae as fine as 96 to the 

 y^Vo^h of an inch, I can only account for this circumstance by 

 supposing a greater difficulty in the case of striae of equal fineness 

 on the j^late, as compared with Amphipleura. This circumstance, 

 which had previously escaped my notice, I have since confirmed by 

 comparison with a number of different objectives. 



In conclusion, I beg you to show the photograph accompanying 

 this note to any of your readers who may take an interest in the 

 performance of objectives of moderate power. I should be very 

 happy to hear from them how this result compares with the work 

 of the best modern EngKsh iths, particularly as I have access at 

 present to no English glass of this jpower constructed within the 

 last two years. 



VI. — Micro-ruling on Glass and Steel. By John F. Stanistreet, 

 F.K.A.S. With Illustrative Eemarks by Henry J. Slack, 

 F.G.S., Sec. K.M.S. 



Plate XCVII. 



By the kindness of John F. Stanistreet, Esq., F.E.A.S., of Liver- 

 pool, I have lately been able to examine some very beautiful speci- 

 mens of ruling on glass, and also on steel, executed by him with a 

 machine of his own contrivance and making, of which some par- 

 ticulars are subjoined. 



The first specimen which I received was a star exquisitely ruled 

 on a small circle of covering glass (yo" diameter), and it cannot 

 be better described than in Mr. Stanistreet 's own words : — " The 

 mounted disk, which I herewith enclose, has a star of 50 rays, or 

 bands of lines, placed radially, each band consisting of 26 lines 

 ruled parallel and equidistant, the ^ oV oth of an inch apart. The 

 star therefore contains 1300 of such lines. The lines are pur- 

 posely scored very strongly to increase the intensity of the diffrac- 

 tion spectra, as I find that hues very much finer and closer (which 

 are just as easy to rule up to about 10,000 to an inch) are not so 

 effective for this purpose." 



Some beautiful diffraction spectra can be obtained with this star, 



VOL. VI. M 



