ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 405 



to have the lines ruled on a double-convex lens of shallow curvature, 

 as the interference with the definition is considerably less than 

 when a glass with plane surfaces is used : with this arrangement 

 Podura-markings can be well shown with a ^ objective. When the 

 binocular is required, a lens without ruling, but of similar curves, 

 should be placed in the other eye-piece to equalize the magnifying 

 jiower in each field. A convenient distance for the lines is -^^ inch, 

 this gives a field not too much crowded with squares, and on the 

 other hand the divisions are not too large to render the setting out 

 of the outline inexact. The drawing is made on ruled paper, the 

 squares being of a size suitable to the intended size of the design, 

 just as in the well-known draughtsman's process of enlarging and 

 reducing by squares. A drawing of any size, from a small sheet to 

 a large lecture diagram, can thus be made directly from the Micro- 

 scope. 



The process also possesses the additional advantage of requiring 

 no change in the position of the Microscope, as is the case with the 

 camera-lucida, and can be used for a long time without any of 

 the strain upon the eye inseparable from the use of instruments, 

 where the image and pencil point are viewed through the divided 

 pupil of the eye. 



With regard to materials, Mr. Suftblk takes exception to the 

 use of JlaTce ivMte for compounding body colours, as in water all 

 pigments made of carbonate of lead rapidly become blackened. 

 Chinese white, a j)reparation of oxide of zinc, should alone be used 

 for this purpose. He also gives the following list of colours which 

 he considers will be found sufficient for nearly every purpose : — 

 aureolin,* yellow ochre, lemon yellow, cadmium yellow, ver- 

 milion, purple madder, raw sienna, burnt sienua, rose madder, 

 light red, brown madder, cobalt, French blue, indigo,! Vandyke 

 brown, blue black, sepia, viridian.| In addition to the colours in 

 cakes, a few that are likely to be used in large quantities should be 

 obtained in tubes ; where thick painting is required, this form of 

 colour is particularly useful. The Chinese white should be kept 

 in a bottle with a greased stopper ; in tubes it soon hardens and 

 becomes unfit for use; it should be worked with the palette-knife 

 and a little water to the consistency required. 



The use of crimson and purple lakes, carmine and all other 

 cochineal colours should be avoided ; the madders are the only 

 safe substitutes. Iodine, scarlet, the chrome yellows, and all aniline 

 colours, should find no place in the colour box. 



Very good efi'ects are obtainable by the use of blacklead, and 



* Aureolin, a transparent pure yellow, quite permanent, and an excellent 

 substitute for gamboge, as, being without gloss, it can be employed in skies and 

 distances. 



t Indigo is only very slowly acted upon by light, and may be considered 

 permanent in the diffused light of an ordinary room ; avoid mixing with Indian 

 red, which speedily destroys it. 



X A transparent oxide of chromium, perfectly permanent, of great use both 

 by itself and in compounding other greens ; the opaque oxide of chromium may 

 also be found useful ; both are extremely permanent colours. 



