218 W. K. KHIDGMAN ON A NEW 



light is of much greater consequence than is commonly supposed, 

 both as regards the distinctness with which minute detail can be 

 made out, and also for the comfort and safety of the observer. It 

 is well known that the yellow light of an inferior flame is very 

 fatiguing, and hence it is often corrected by blue glass ; but 

 although right in principle, it is mostly a failure through being over- 

 done with the blue, producing the cold indistinctness of moonlight, 

 but which lias also against it other serious objections. Under the 

 actinism of the blue ray seeds germinate in a much shorter time 

 than their natural period ; but they speedily die a premature death 

 from being unable to grow under its effects. Plants similarly 

 circumstanced produce neither leaves nor flowers, for want of the 

 yellow or luminous rays which effect the elimination of their sap. 

 The chemical action is unbalanced, and is all one way — that is, it is 

 unipolar, and the chemical life of the plant differs but little from the 

 chemical life of the eye, and hence, under bright blue glass spectacles, 

 the frequent cases of inflammation are readily accounted for. But 

 whilst the blue rays are thus detrimental from their powerful 

 chemical effects, the next colour in the spectrum — the yellow — is 

 equally remarkable for their absence, and it is this absence which 

 allows the photographer to manipulate his chemicals, unaffected, 

 under its influence. Now if we arrange a micropolariscope and a 

 revolving selenite so that we can get a succession of tints from the 

 green to the yellow, we shall find that as the blue rays of the green 

 decrease in quantity, and are ultimately lost in the yellow, there 

 is one particular point at which neither the green nor the yellow is 

 clearly appreciable, but that a soft neutral tint only appears, and any 

 delicate test being submitted to this light, it will be found to be 

 clearly discernible without effort and without glare. By this admix 

 ture of the two opposite conditions, the actinic and luminous rays, 

 we produce a perfectly balanced light, which is in every way all that 

 can be desired. 



If pure gold be alloyed with about a fifth part of its weight of 

 silver, it then forms the green gold of the jeweller, but if to some of 

 the silver of commerce be added a small proportion of gold, it will 

 form a tolerably hard alloy, capable of taking a high polish, and 

 giving a beautifully soft and mellow light of the desired quality. 



Arr-ENDix. — As it is an essential point to have a well-defining 

 polish upon the speculum, it may not be out of place to offer a sug- 

 gestion as to how this can most readily be obtained. My experience 



