484 



plates and " flats," exhibiting Newton's rings, were also shown. 

 For the further benefit of members, Mr. Watson Baker had pro- 

 vided a short " Explanation of the Demonstration" in the form of 

 a specially printed leaflet, from which we extract the following : 

 " The successful production of a microscope objective is dependent 

 on (1) the mathematician, (2) the tools, (3) the workman, (4) the 

 mounter. The mathematician so computes an objective that, 

 when it is constructed from the ' optical glasses which he has 

 chosen as suitable for their refractive and dispersive qualities, 

 with the component lenses shaped to the thickness and curvature 

 he has prescribed, and finally mounted and accurately centred at 

 the computed distances, it will exactly realise his intentions. 

 The glass being selected, convex and concave templets or gauges 

 are made of the radii of the various constituent parts, and pairs 

 of tools turned to fit the curvature of the gauges. These tools 

 are divided into roughers, true- tools, and polishers. From the 

 glassworks the glass is received in thick slabs or plates, and by 

 means of a slitting-machine, consisting of a rapidly rotating iron 

 plate charged with diamond-dust and oil, thin plates are cut off 

 to approximately the thickness of the lens. These plates are then 

 cut into small squares, and are trimmed nearly round with hand- 

 shanks. The little rough disc of glass is then cemented with 

 shellac to a holder, and rotated in a lathe, and, with a steel tool 

 lubricated with water, is edged to within a fraction of its ultimate 

 diameter. The next stage is to shape the face spherically. The 

 lens in its rough state is then removed, in its holder, from the 

 lathe, and the roughing tool takes its place. The lens, in its 

 holder, is held in the hand ; the lathe is rotated, and the lens is 

 ground against the required tool ; emery moistened with water 

 is the abrasive agent employed. Polishing is then done with a 

 tool lined with a composition consisting largely of pitch, rouge 

 and putty-powder being among the polishing materials employed. 

 From time to time the curve is tested by means of a ' proof -plate.' 

 A ' proof-plate ' is a plate of glass which has been worked so as 

 to precisely fit the exact curve of the lens which it is intended to 

 test by its means. If a carefully cleaned lens be brought into 

 contact with the curve in the ' proof -plate,' and is nearly correct, 

 there will be observed the phenomena known as Newton's Rings. 

 These coloured rings are produced whenever two reflecting surfaces 

 are brought very close together. These rings form an extremely 



