TOPICS OF INQUIRY, ETC. 83 



transient, is present, and when absent. A burning coal 

 gives light, but loses it instantly when strongly compressed ; 

 the crystalline humour of the glowworm, after the worm s 

 death, even when broken and divided into parts, retains 

 light for a short time, which however soon after fades 

 away. 



in. Table of Degrees. 



We must remark which sorts of light are more intense 

 and vibrating, which less : the flame of wood produces a 

 strong light; the flame of spirit of wine, a weaker; the 

 flame of coals when fully kindled, a very dim and scarcely 

 visible light. 



iv. Colours of Light. 



We have to consider the colours of light, what they are, 

 what not : some stars are white, others glittering, some red, 

 some lead-coloured ; the common sorts of flame are gene 

 rally croceous, and among these the coruscations from the 

 sky, and the sparks from flint, tend most to whiteness ; the 

 flame of sulphur is ceruleous and beautiful ; but in some 

 substances are purple flames. No green flames are ob 

 served : what most inclines to greenness, is that of the glow 

 worm. Neither are there any crimson flames discovered : 

 heated iron is red, but if heated somewhat more intensely, 

 it becomes as it were white. 



v. Reflections of Light-. 



We have to observe what bodies reflect light : as mirrors, 

 water, polished metals, the moon, precious stones. All 

 liquid bodies and such as have very equal smooth surfaces 

 are somewhat bright : but brightness is a certain small 

 degree of light. 



We have to remark attentively, whether or not the light 

 of one lucid body can be reflected by another lucid body; 

 as if you took heated iron and opposed it to the sun s rays. 

 For the reflections of light are reflected on, yet becoming 

 gradually feebler, from mirror to mirror. 



v i. Multiplication of Light. 



The multiplication of light must next be considered : as 

 by mirrors, perspectives, and the like, by which light may 

 be sharpened and thrown to a distance, or also rendered 

 subtler and softer for distinguishing visible objects ; as you 



