ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 637 



fusion may be taken from the pots (placed in tbe furnace for that purpose) 

 with a platinum brush, the handle of which is hollow, and through which 

 ignited air and gases are caused to pass into the wires of the brush, so 

 that the matter being kept at the required temperature has not time to 

 solidify, and may be laid upon the lens placed upon the preceding 

 apparatus, when stationary, in the same manner as a layer of any other 

 substance, such as i^aint, would be laid on. Plates of suitable thickness 

 and forms in crown and flint glasses may also be prepared by blowing 

 and moulding, as hereafter described, and caused to adhere together; for 

 this purpose the arrangement of tubes t, t^ (fig. 26) is employed, fed by 

 air and hydrogen gas or any other combustible giving a flat fan-shaped 

 flame. Pincers having two or three jaws are held in each hand for the 

 purpose of holding the plates to be united ; when the faces to be united 

 have been softened they are brought in contact through the flame, the 

 pincers being continually kept turning. In the case of periscopic convex 

 plates, the one which is to take the form of the other must be softened 

 on both faces. The handles of the pincers and also that of the platinum 

 brush have a tube like that of the blow-pipe c for the passage of air and 

 gas which passes through them, the flame impinging upon the back of each 

 plate at the same time that the flame of the intermediate burner impinges 

 upon the two faces ; this arrangement allows of one of the plates being 

 sufficiently softened to take the form of the other. 



" If the plates are of somewhat large dimensions the pincers are mounted 

 upon a lathe to the mandrils of which a rotary motion of greater or less 

 speed is imparted, and also a reciprocating motion. One of these shafts 

 may advance one of these two plates upon the other ; seams and in- 

 equalities are caused to disappear by the softening of the glass combined 

 with the motion, but if the lenses to be produced are of large dimensions 

 the preceding processes might be partly insufficient, and in that case plates 

 are employed of the dimensions, forms, thickness, and refractive and dis- 

 persive powers suitable for the effect desired to be obtained. They are 

 laid over one another one by one, and set in a mould of polished clay which 

 is introduced into an annealing oven and left there until the plates all 

 adhere. If the lens is to be of any other form than plano-convex the 

 mould has a heavy cover glazed inside, which bearing upon the lens, 

 imparts to it the form (either convex or concave) which it has itself. A 

 greater or less degree of pressure may be employed in order to expedite 

 the adherence and increase the density. If, seeing the various kinds of 

 glass (crown and flint) employed fluid or in plates, and seeing the curves 

 of the lens and the length of focus which might be required, achromatic 

 lenses free from aberrations of any kind could not be obtained, groups 

 might be employed formed of layers superposed and afterwards united as 

 simple plates. If required, fluxes might be interposed between the plates 

 or upon the exterior surfaces of the lenses with the platinum brush. These 

 fluxes may be composed as follows : — One part of white sand, three parts 

 of minium, • 5 of calcined borax, or three parts of white sand, one part 

 of minium, and five parts of calcined borax, or others, according to the 

 effect desired to be obtained. Instead of these fluxes pulverized glass 

 (either flint or crown) is employed especially upon exterior plates, by 

 means of the fan-shaped burner; this powder is brought into a state of 

 fusion, and the required form is given to it by a suitable movement. 

 Bevelled plates which are partially superposed, or concentric circles and 

 plates, the bevels of which overlap, may be employed, so that the index of 

 refraction shall vary from the centre to the edge. Instead of these circles 

 or plates, or concurrently with them, annular parcels or fagots of glass 



