88 REPORT — 1876. 



1 say " almost any want of homogeneity," because, with one exception, I believe 

 any kind can be detected ; but I have met, very rarely, instances of one peculiar 

 class of this defect which it is not possible to detect till the disk is actually worked 

 into an objective : this happens when a slight gradual change of density occurs be- 

 tween two portions of the disk with no abrupt line of separation between. 



Now this process, though a very simple one to a practised eye, is by no means so 

 to an uneducated one ; and I have often desired a method by which I could gi-aphi- 

 cally represent those faults so that I might be able to communicate to others my 

 ideas as to their exact forms and appearance, position in the disk, and so forth, and 

 also to form a record of them. This, by a very simple contrivance, I have succeeded 

 in doing, and I am now able to photograph these defects in optical glass with per- 

 fect certainty. 



A glance at the diagram will sufRce to show the principle by which this is 

 effected. 



Fig. 2. 



The eye in the first instance (that of eye observation, fig. 1) is replaced in the 

 second case (fig. 2) by a photo-camera; and, with a little care in adjusting the 

 image of diaphragm illuminated by a lamp on the diaphragm of photo-lens, very 

 excellent photographs can be obtained. In fact the stop of the lens replaces the pupil 

 of the eye, the photo-lens the crystalline lens, and the sensitized plate the retina. 



The defects arising from want of homogeneity in optical glass may be divided 

 into three classes : — 



1. Threads, or fine seams of some different quality of glass passing through the 

 otherwise homogeneous disk, sometimes insignificant, sometimes long, but very 

 rarely of any width. These are of but little importance. 



2. Veins, or syrupy bands. These are portions of glass of differing and various 

 densities not properly amalgamated together. Their appearance is that produced 

 by adding a strong syrup solution to water. The forms of these veins are sometimes 

 very fantastic. 



This form of defect is very detrimental to the proper performances of the glass. 



.Srd. Sometimes, but very rarely (only four times in my experience), have I met 

 with disks of glass having a density slightly different in different parts, without any 

 well-defined line of demarcation between the different parts. This is most destruc- 

 tive to its performance as an objective, and a most dangerous fault; for whereas in 

 the two former cases the defects can be easily detected and even photogi'aphed, this 

 third defect defies detection until the disks be formed into an objective. 



It is fortunate for opticians that this last defect is of such rare occurrence. 



The extreme usefulness of this simple device for photographing the defects in 

 optical glass is self-evident. 



In the first place, faults can be detected by those whose eyes have not been suf- 

 ficiently educated to perceive them by the old method ; a record can be made of 

 any remarkable defects ; their appearance and form can be graphically repre- 

 sented and described ; and, lastly, it can be ascertained by this process whether the 

 veins are closer to one or other surfaces and are capable of being removed by ai-ind- 

 ing, a point which is very difiicult indeed to ascertain otherwise. This last infor- 

 mation is obtained by photographing the faults and then grinding off" a small quan- 

 tity, and rephotographing and comparing the photographs to see if any parts have 

 disappeared. Many other useful pui-poses seem to be too self-evident to require 

 mentioning. 



