7V8 



CAUSES OF EEEOES IN OBSERVATIONS. 



of minute pearly globules, hung across the field of view (fig. 952 a). 

 The second in point of remarkableness, and the farthest of the four 

 from the eye, consists of watery-like threads, destitute of any globular 

 appearance, and depending chiefly from the upper part of the field 

 (fig. 952 b). I caU the former the pearly spectrum, and the latter the 

 watery spectrum. In two distinct planes, between those occupied by 



these two spectra, float two 

 sets of globules, not aggre- 

 gated into threads, but insu- 

 lated. These constitute what 

 I call the insulo-globular 

 spectra. The individual glo- 

 bules of the set farther from 

 the eye, being hazy and Hi- 

 defined, may be compared in 

 appearance to small grains of 

 sago (fig. 952 c). The globules 

 of the set nearer to the eye 

 are clear in the centre, ex- 

 teriorly to which they present 

 a sharp black ring, and stOl 

 more exteriorly a lucid cir- 

 cumference (fig. 952 d). 

 These four sets of spectra 

 never mingle with one ano- 

 ther, so as to change the order in which they stand before the eye ; 

 but the pearly spectrum always appears the nearest ; then the sharply- 

 defined insulo-globular ; then the obscurely defined globules ; and 

 farthest away the watery threads. 



" Almost every eye, even the most healthy, and which has never 

 attracted the possessor's attention by muscse volitantes, exhibits the 

 pearly spectrum, on being directed towards a luminous field, through 

 a fine pin-hole, the eye-glass of a compound microscope, or a convex or 

 concave lens of short focus. I have given it the same name of the 

 pearly spectrum, from its resemblance to a string of pearls. Prevost 

 had already called it apparence perUe, or simply perles. 



" The lines of the pearly spectrum are hung across the field of 

 vision as often transversely as vertically. On first directing the eye 

 towards the luminous field, in one or other of the methods just men- 

 tioned, perhaps only a very few small pearly globules are perceived ; 

 but after steadily regarding it for a short time, numerous strings of 

 them are discovered, generally twisted in difierent forms, and present- 

 ing a variety of knots, loops, and agglomerations. Sometimes they 



' Fig. 952. Four sets of spectra, which are apt to cause errors iu ohsei*vation3 with the 

 microscope. 



Fig. 952. 



