28 HOW TO WORK 



of measurement may be appended to them by proceeding in the 

 manner described in fig. 63. 



Mr. Conrad W. Cooke, in 1865, designed a new instrument for 

 drawing which he terms a " micrographic camera." By this instrument 

 an image can be thrown on a sheet of paper placed in a horizontal 

 or slanting position, so that anyone may trace on the paper the out- 

 lines and detail with a fair amount of accuracy. It is useful also for 

 purposes of demonstration, for two or more persons may at the 

 same time conveniently examine the image formed on the paper. 

 The head of the observer is isolated from external light by means 

 of a curtain which falls over the back of his chair. Measurement 

 of the objects shown in this camera may very easily be made, and 

 boxwood scales corresponding to the magnifying powers of the 

 different objectives are furnished. All the necessary adjustments 

 can be effected from the inside, in order to avoid the inconvenience 

 to the observer of continually altering his position. The use of 

 the microscope is not entirely confined to the examination of 

 transparent objects, for an image of many of the opaque prepara- 

 tions may be shown with it on the paper. The effects of dark 

 ground illumination (with the paraboloid and other instruments) 

 and those of the polariscope may be shown on the paper without 

 loss of definition ; and all these accessories, as well as the objectives 

 used, are the same as those of a microscope of the ordinary con- 

 struction. The whole apparatus is made to fold up so as to 

 occupy as little space as possible, for the sake of portability. I 

 believe the arrangement, which, however, is not adapted for high 

 magnifying powers, can be furnished by Mr. Ross. 



4G. Of making Drawings which it is intended should be Engraved. 

 With a little practice, the observer may acquire the power of 

 drawing on wood, and the engraver will often be able to produce 

 a more faithful representation of the object than he could by 

 copying the drawings of the microscopical observer. The drawing 

 should first be made roughly on paper, in order to obtain the 

 size and general characters of the object. A piece of retransfer 

 paper is then placed upon the prepared block, and the prominent 

 lines of the drawing traced with some blunt-pointed instrument (a 

 needle, the point of which has been made slightly blunt by filing it, 

 answers very well). By using a slight pressure, the colour of the re- 

 transfer paper is transferred to the wood block in the lines corres- 

 ponding to those of the drawing. These lines are afterwards 

 reproduced by lead pencil, corrected, if necessary, and the delicate 

 parts of the drawing filled in by carefully copying from the 

 object. 



