2 40 Intelligence and Miscellaneous Articles. 



raising success. I had in my possession a spectrometer by Brimer 

 of Paris — his small model, similar to the one used by Mascart, and 

 figured in his paper on the measures of wave-lengths. This in- 

 strument has a good steel centre, and was furnished with a circle 

 divided on silver, reading by means of the verniers to 10". The 

 diameter of the circle is small, not quite 7 inches ; and the inability 

 to read smaller angles has always been its weak point. I have 

 substituted for this metallic circle one of glass about 10 inches in 

 diameter, divided by Mr. Stackpole to ten minutes of arc, and read 

 by two micrometer-microscopes magnifying seventy-five times ; each 

 revolution of the screws being equivalent to one minute, the drums, 

 being divided into sixty parts, read to seconds with easy estimate 

 of fractions ; each degree-line is numbered so as to be visible in the 

 field of the microscope. I was able to furnish to Mr. Stackpole a 

 well-tried diamond which has made lines of the greatest delicacy, 

 being much finer, as seen in the microscopes, than the spider-lines, 

 by means of which the bisections are made. The advantages of this 

 system are obvious, viz. : — perfection of surface, permitting a line of 

 any desired fineness ; facility of illumination, permitting the exten- 

 sion of the power of the reading-microscopes to several hundred 

 times ; smallness of dimensions, and consequent cheapness and 

 avoidance of almost all the questions of flexure and local effects 

 of temperature. 



I am convinced from the ease with which one second is read on 

 my instrument, with microscopes only 4| inches long, including 

 objectives and eyepieces, that, upon a circle of 15 inches provided 

 with powerful microscopes, greater precision could be attained in 

 the reading of angles than with the largest metallic circles now 

 in use. 



For the purpose of showing the degree of precision attainable. I 

 add two series of bisections of lines on the circle made by myself, 

 and two made by a lady, marked respectively E/. and M. : — 



R. E. M. M. 



7-5 



1-8 



23-6 



11 



7-4 



1-3 



23-6 



11-2 



7-7 



1'9 



23-3 



11-8 



7-4 



1-8 



23-8 



11 



73 



1-8 



24-5 



10-9 



7-6 



1-8 



23-9 



11-5 



7-7 



1-9 



23-7 



11-5 



7-4 



2 



24 



11-5 



7-6 



2-3 



23-9 



11 



7-8 



2-3 



24-3 



11-4 



Mean.. 7-54 1-89 23-86 11-28 



It will be readily seen that the probable error of any single 

 reading in any one of these series is considerably less than half a 

 second, while the probable error of the mean of any series is a 

 much smaller fraction. — Silliman's American Journal, August 1876. 



