702 Transactions of the Society. 



In this drawing (fig. 121) of the lines of a fine stage-micro- 

 meter, with drawing-board horizontal and inclined, all appreciable 



Fig. 121. 



distortion has been got rid of by the simple device of raising 

 the right-hand extremity of the drawing-board. Well, this dis- 

 tortion being eliminated, and the loss of sight of the pencil under 

 certain conditions condoned, we must recognize this prism of 

 Nachet as being exceedingly useful and convenient in practice, it 

 being so hinged upon the collar which clasps the ocular, that the 

 optical part can be thrown back like a hood, so as to give a clear 

 view of detail through the ocular only, and the prism can be tilted 

 forward again at will to resume the sketching of outlines with no 

 fear of loss of coincidence with its former position. A tinted glass 

 which can be interposed between the object and pencil respectively, 

 helps to make the Nachet hooded prism a great favourite of mine. 

 Speaking of this " light-moderator " brings me to the point 

 that one of the great secrets of success in prism drawing from the 

 Microscope is the equahzation or balancing of Ught from object 

 and pencil. The best effect by far is got by two lamps. Where 

 light from the paper is too glaring, as will often happen with 

 the Schroder prism used in dayhght, the half-shadow of a curtain 

 allowed to fall upon the paper conduces wonderfully to ease in 

 sketching. I have intimated that with the Schroder prism you may 

 inove your head as much as you like, but not so with the other 

 little optical appliances, and this keeping the head steady is as 

 difficult as it is wearisome. Failing an appliance something like 

 a photographer's " head-rest," let me suggest a substitute in a 

 microscope-box, in the position that the left elbow can rest upon it, 

 when the outspread thumb and fingers placed against the forehead 

 will be found to keep the head of the draughtsman fairly steady. 

 A small black velvet curtain, so hung as to touch the microscope- 

 tube just below the ocular, will be found to aid materially in 

 distinctness by cutting off diffused light. You want to see all you 

 can of your object, but make up your mind you will never see 



