342 MICROMETRY. 



of the coil, and then by counting the number of threads found 

 the diameter of the wire. This was then cut into small pieces, 

 and one of them was placed upon or near the object to be mea- 

 sured, and a comparison made. That this method was very crude 

 is evidenced by the fact that Jurin found the diameter of a human 

 blood corpuscle to be i /1940th of an inch. 



In 1742, Benjamin Martin described an eye-piece micrometer 

 which consisted of a screw having fifty threads to the inch. One 

 end of the screw was pointed, while the other carried a hand 

 which passed over a dial divided into twenty parts. This appliance 

 doubtless gave fairly accurate results, and in a more or less modi- 

 fied form has appeared in later micrometers. Baker also credits 

 Martin with a glass micrometer having parallel lines i/4oth of an 

 inch apart, and placed in the focus of the eye-piece. 



Geo. Adams, Sr., in his Micrographia, 1746, says that, acting 

 upon the idea suggested by Jurin, he constructed a micrometer 

 which consisted of small silver wires stretched in the form of a 

 lattice, the distances between the wires being equal to the diameter 

 of the wire, which was probably from the i /500th to the i /600th 

 of an inch. This lattice was used upon the stage, being placed 

 under the object if it was transparent and over it if opaque. 

 Adams, however, appreciated the difficulty of measuring objects 

 which were less than the diameter of the wire. 



Three years prior to Adams' publication. Baker described a 

 similar arrangement invented by a Dr. Smith, consisting of a lattice 

 of fine wires or small squares drawn upon glass with a diamond. 

 This, however, was designed as an aid in drawing microscopic 

 objects. According to Baker, Cuff devised a micrometer in 1747 

 which consisted of a lattice of fine wires i/5oth of an inch apart, 

 set in a circular frame to be placed in the focus of the eye-piece. 

 This device is shown in Fig. i, PI. XV. Baker made a similar 

 micrometer of human hairs placed i/ioth of an inch apart. Fig. 2. 



Geo. Adams, Jr., in his Essays on the Microscope, describes a 

 needle micrometer which he states was first made by his father. 

 This is shown in Fig. 3. It was designed to be clamped to the 

 tube of the microscope, the needle passing through a small open- 

 ing in the eye-piece. The screw had fifty threads to the inch, 

 and was provided with an index moving over a circular plate 



