On Drawing Prisms. By Dr. Anthony. 701 



For the Microscope placed vertically, I will only call attention 

 to one drawing appliance, viz. the " Nachet hooded prism," which 

 you will see I have placed on the ocular of a Microscope in the 

 usual position for sketching. Looking through this prism the 

 image in the Microscope will he seen projected some 5 in. on to 

 the drawing-hoard placed on the right-hand side. As a prism 

 this has all the advantages and the faults of the class to 

 which it belongs ; a very prominent fault being the all but total 

 loss of sight of the image of the pencil when an attempt is made 

 to follow the outline of an object seen between the centre and the 

 outside edge of the field of view — calhng the outside that which is 

 apparently farthest away from the microscope-body. 



If the drawing-board is placed flat upon the table, you will find 

 that your drawing so made by the Nachet will be much distorted. 

 A good article in our Journal * set me off to experiment on this 

 distortion, and how to get rid of it. I show the satisfactory results 

 arrived at. The boy's head — a cutting from ' Punch ' — has been 

 copied twice, in the left-hand picture the drawing-board was flat 

 upon the table. Ihe eye wiU detect the distortion in a moment, 

 in the head being far too deep from front to back. The right- 

 hand image— taken with the board raised 2 in. at its right-hand 

 end — shows not a trace of distortion. Here is a still more severe 

 test: these (fig. 120) are copies of the circles in Moller's smaller 



Fig. 120. 



" typenplatte " under the same conditions as to position of drawing- 

 board. The right-hand picture is shown by the dotted circle struck 

 by the compasses to have lost the distortion, which is painfully 

 evident in the one on the left-hand side. 



Vol. III. (1883) p. 560. 



