Royal Microscopical Society. 3 



best. The miniature image formed in the focus of the microscope 

 on the stage was reduced six times exactly by means of an objective 

 placed below it in a certain position within a sliding tube, adjusted 

 precisely by a stud. He has thus brought the arrangement to this 

 point, that, with an old ^-inch object-glass, the miniature aerial 

 image of the micrometer spider-lines appeared precisely one-sixth 

 of their natural size. In other words, the instrument is rendered 

 six times more sensitive than it was before. There was not the 

 slightest difficulty in moving the aerial image or spider-lines some- 

 thing like 80000th of an inch at a time in the plane of focal vision 

 on the stage. When an observer had a very delicate object which 

 he wished to measure, it was extremely inconvenient to disarrange 

 the microscope. By this simple method, the micrometer was always 

 in use ; it formed part of the condensing apparatus, and by turning 

 a screw the micrometer lines rose into view, which can then be 

 traversed with the greatest nicety. 



In measuring very minute quantities, it had been always a 

 matter of difficulty to work beyond a certain point, in consequence 

 of the vibration which was attendant on any movement of the 

 instrument; but if you moved this micrometer placed below the 

 stage, vibration would be avoided. On examining microscopically 

 the image under a power of 200 by means of a stage micrometer, 

 you are able at once to see, with the greatest precision and accuracy, 

 that you really have got a certain proportion between the micro- 

 meter lines and the aerial image. He had chosen six times because 

 it happened to suit the size of his instrument. If such as the J be 

 used the image is reduced twelve or fourteen times. As a general 

 rule it is found the ^ inch of low angle gives sufficient light. It gave 

 a sharply-defined miniature of the spider-lines ; and the 100th of 

 an inch interval between the aerial lines under a ^ inch was a very 

 considerable quantity to look at. It could then be divided easily 

 into 600 parts. 



There was some interest displayed at the present moment in 

 ascertaining magnifying power, and it was rather important to know 

 what one was about. Very great difficulty is experienced in using 

 the old-fashioned glass micrometer, usually placed at the stop within 

 the eye-piece ; there were disturbances arising from four surfaces of 

 refraction, and from various changes in the Canada balsam, which 

 underwent decomposition. The writer wishes now to communicate 

 a plan which perhaps will be generally adopted, and it is this ; 

 instead of using a micrometer glass of that nature, after some little 

 study he had had lines engraved upon a very long focus plano-convex 

 lens.* The lens was then inserted into the stop. Its magnifying 

 power was of course so slight at that position that it did not appre- 

 ciably alter the effect of the eye-piece. By this means he got per- 

 * The weakest that is made for spectacles : nearly plano-convex. 



B 2 



