68 OPTICAL PROJECTION 



before we can obtain the greatest illumination the lime-light 

 is capable of affording. These hints are the result of long 

 and sedulous attempts in that direction, with a view especially 

 to microscopic projection. There will remain still what 

 scientific observers term 'personal equation.' With the 

 very same apparatus, two men will not get precisely equal 

 results. I have always found that if unusually fatigued, I 

 could never myself get the same light, by a perceptible 

 quantity. A steady and delicate hand in adjusting the taps 

 has much to do with this, and there are differences in this 

 respect between different people. 



35. Limes. While very soft ' chalk ' limes give rather a 

 better light with oxy-calcium jets, the very hardest are 

 needed for the mixed. ' Nottingham ' and ' Excelsior ' limes 

 are both well known. Both vary somewhat in quality ; the 

 best of both are excellent, but the Excelsiors are apt to 

 crack across under powerful jets. The other does not, but 

 only lately has hard lime like ' Nottingham ' (which really 

 I believe comes from Ireland) been obtainable turned true, 

 and this for some work is essential. 



36. Substitutes for Limes. Many attempts have been 

 made to avoid the turning necessitated by the pitting of the 

 lime. Magnesium oxide made into a paste with water, dried, 

 and then exposed to gradually increasing heat, is said by 

 Dr. Eoux to stand the jet for hours ; but he cannot have 

 used jets of much power, for I find no superiority whatever 

 over lime, though the light is very fair. A mixture of 

 calcium sulphate and magnesium oxide, which has been 

 recommended, is no harder, but I have sometimes thought 

 either this or the former might behave better if silicated 

 by mixing with solution of some silicate instead of water. 



The only material hitherto used which really stands the jet 

 is zirconium oxide. This was stated many years ago by 

 M. Du Motay to be 'the most luminous' as well as most 

 refractory of all substances, and the statement has been 



