122 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



Fig. 12. 



position, on to a hot plate, and drop a fragment of sulphur on the 

 lens, raise the heat till the sulphur melts and flows round the edge. 

 By capillary attraction it runs into the joint. Although the melted 

 sulphur is very fluid, yet it has a singular disinclination to attach 

 itself, or spread on a polished surface of glass ; and this property quite 

 prevents it from creeping over the back of the lens, as balsam or 

 shellac will do, and when cold the button of sulphur on the front may 

 be easily picked off with a needle point, leaving the surface of the 

 glass clean. 



" Fronts extending beyond the hemisphere have been proposed by 

 Professor Stokes. I do not at present pretend to give a comparison 

 of any degree of advantage or superiority over the common hemi- 

 spherical front, as I have not yet tested the relative performances of 

 both, as applied to the same object-glass. I have first to state that 

 there is no great difficulty in constructing such lenses. In fact, from 

 the mode that they can be worked in the mould, the figure of this 

 form of lens is more likely to be perfect than a hemisphere. Having 

 ascertained the thickness to be given from the flat face to the top of 

 the sphere, a piece of glass polished on one side is flattened down to 

 the thickness, then cemented on to a suitable chuck, on which the 

 finished lens will appear, as in Fig. 12. In working and polishing, the 

 sphere may be rolled round in the tool till the 

 holding chuck and handle is carried out at right 

 angles to the axis of the mandril of the lathe. 

 We cannot go to this extent with a hemispherical 

 lens without working off the edge too much, but 

 the more a sphere is turned about in every direc- 

 tion, the more perfect the figure becomes. 



" The difficulty has hitherto been to mount 

 these balloon lenses securely. The plan that 

 has been adopted is to set them on a plate of thin 

 cover-glass, with Canada balsam or other cement, 

 and burnish the plate in at the edge. The 

 whole arrangement is thus so fragile that, after 

 the balsam is hard and brittle, the mere act of 

 wiping the front will probably start the surfaces, 

 and the least pressure on the object-slide (which 

 even the most careful manipulator cannot always 

 avoid) will dislodge the lens, besides the objec- 

 tion of somewhat impairing the finest definition 

 by two additional surfaces, even when cemented. 

 I have seen a great number of instances wherein 

 the old triple fronts made by Eoss, as in Fig. 13, in order to gain 

 the effect of the full hemisphere, have had the thin flint disk broken 

 in by contact with the slide. In mounting the balloon front, I substi- 

 tute a disk of well hammer-hardened metal, in lieu of the glass-sup- 

 porting plate, as shown by Fig. 14. After the lens is polished, it is 

 removed from the holder and its convex side cemented on to a ring- 

 chuck with shellac, and the flat surface set true by the ' candle-flame ' 

 test. This can be quickly and easily done by warming the chuck with 



