244 USE OF THE MICROSCOPE. 



will those also in which the thickness of the crystal is not 

 below a certain standard, this for the selenite is the "00046 of 

 an inch ; the red colour is always produced by the thickest 

 films, the violet by the thinnest. It must, however, be borne 

 in mind that the red and green are always complementary to 

 each other, or together make white light. 



The exhibition on a screen of the coloured rings in crystals 

 cut at right angles to their crystaUographical axes, was first 

 effected by Mr. Woodward, by the gas polariscope; the 

 crystal having been placed within the focus of a lens of low 

 power, and a tourmaline used as an analyzer. Mr. Legg 

 has successfully effected the same results by the achromatic 

 compound microscope, and in order to afford an additional 

 means of investigating these phenomena, he recommends the 

 use of the erector, before described at page 126, by which the 

 microscope is converted into a telescope of low power. When 

 the Mcol's prism is used as a polarizer, the field of view will 

 be too much limited for the action of the erector; in these 

 cases it will be advisable to employ a bundle of thin glass plates, 

 placed in such a manner that light may fall on them at an 

 angle of 56" 45', when the light reflected will be polarized, 

 and a large field illuminated, but not with so much brilliancy 

 as by the prism and concave mirror. 



The usual mode of exhibiting microscopic objects by polar- 

 ized light is to place them on the stage of the instrument 

 with a Mcol's prism as a polarizer, adapted below the stage, 

 and a similar prism above the eye-piece; in this way most 

 crystalline bodies may be shown. Some few vegetable struc- 

 tures may also be exhibited in the same manner, amongst the 

 latter may be enumerated the hairs on the leaf of the Eloeagnus, 

 the siliceous cuticle of the Equisetum, and of some of the 

 grasses, together with starch of various kinds, all of which 

 are beautiful objects for the polarizing microscope. Many 

 animal structures, such as feathers, slices of quill, horn, hoof, 

 and other cuticular appendages, are best shown by placing a 

 thin film of selenite or mica beneath them, by which they 

 become intensely coloured ; the selenite or mica should be of 

 uniform thickness, in order to develop the true structure of 



