Application of the Microscope to the Art of Design. 113 



Before leaving tlie subject of microscopic colour study, let 

 us point to the use of the polariscope for observations, which 

 no other method can place so readily within reach. It is usual 

 to display polarizing objects in their most striking situations, 

 when they present contrasted masses of pure prismatic colour. 

 From these, however, the decorative artist will learn little ; but 

 if he takes a concentrated solution of nitre, or tartaric acid, 

 and allows a drop to crystallize rapidly on a warm slide, he 

 is tolerably certain to obtain his material in such forms, and 

 in such varying thicknesses, as will enable him to produce a 

 number of interesting tertiary combinations, by adjusting the 

 polarizing and analyzing prisms to the best positions for the 

 particular effect desired. These experiments require a selenite 

 stage, and the means of rotating both prisms, and not the 

 polarizer only, as is the case with the arrangements that some 

 opticians send out. 



For a combination of green tints and forms, adapted to 

 the jeweller and enameller, the desmids may be recommended, 

 of which some useful specimens are figured in Recreative Science, 

 vol. ii. p. 279. The beautiful fluted and otherwise marked 

 bottles of the Foramenifera fern, called Lagense, would furnish 

 classical patterns for the glass-blower, and his attention should 

 likewise be directed to the polycystina from Barbadoes, whose 

 siliceous shells of varied shapes glitter like the finest crystal 

 when lit up by the dark ground illumination which the para- 

 bola affords. 



Another class of objects that merit attention for the sugges- 

 tions they afford, are the spines of the echinus, or sea urchin. 

 The sea urchins belong to the echinoclermata, or c ' hedge-hog 

 skinned animals," a class which comprehends star-fishes, sea 

 hedge-hogs, or urchins, and those curious creatures the sea 

 cucumbers. Many readers will be familiar with Edward Forbes' s 

 classical work, entitled "British Star-fishes;" but for the benefit 

 of those who do not know his remarks on the urchins, we may 

 state that in one of moderate size he found 3720 pores arranged 

 in ten series, or "avenues," with one sucker to every two pores. 

 Their shells are composed of nearly 600 angular pieces fitted 

 together like a mosaic, each plate being enveloped in the lining 

 membrane, by which it was deposited, and which provides for 

 its growth. These plates are furnished with about 4000 spines, 

 every spine being built up of a multitude of pieces deposited by 

 a living tissue, and producing a radiating pattern as shown in 

 the tinted plate, which has been engraved from a drawing 

 by Mrs. Henry Slack. Fig. A represents a thin section of an 

 usually beautiful spine, illuminated by a unilateral slanting 

 light, arranged at such an angle as to mass certain portions 

 together in solid diverging rays. Fig B gives a truer idea 



