T.HE USE OF THE CAMEEA LUCID A 287 



This camera may be used with a hand-magnifier, or with simple 

 lenses used for dissection and other purposes. 



With one or other of the foregoing contrivances, every one may 

 learn to draw an outline of the microscopic image ; and it is extremely 

 desirable for the sake of accuracy that every representation of an 

 object should be based on such a delineation. Some persons will use 

 one instrument more readily, some another, the fact being that 

 there is a sort of ' knack ' in the use of each which is commonly 

 acquired by practice alone, so that a person accustomed to the use 

 of any one of them does not at first work well with another. 

 Although some persons at once acquire the power of seeing the 

 image and the tracing point with equal distinctness, the case is more 

 frequently otherwise ; and hence no one should allow himself to 

 be baffled by the failure of his f rst attempt. It will sometimes 

 happen, especially when the Wolfaston prism is employed, that the 

 want of power to see the pencil is due to the faulty position of the 

 eye, too large a part of it being over the prism itself. When once 

 a good position has been obtained, the eye should be held there as 

 steadily as possible, until the tracing shall have been completed. It 

 is essential to keep in view that the proportion between the size of 

 the tracing and that of the object is affected by the distance of the 

 eye from the paper ; and hence that if the microscope be placed 

 upon a support of different height, or the eye-piece be elevated or 

 depressed by a slight inclination given to the body, the scale will be 

 altered. This it is, of course, peculiarly important to bear in mind 

 when a series of tracings is being made of any set of objects which 

 it is intended to delineate on a uniform scale. 



A valuable adjunct to a camera lucida is a small paraffin lamp, 

 seen to the left of plate III., which illustrates the correct method 01 

 using the camera lucida. This lamp is simple, and is capable of being 

 raised or lowered, fitted with a paper shade, for a great deal of the 

 success attendant on the use of the camera depends on the relative 

 illumination of the microscopic image on the one side, and of the paper 

 and fingers and pencil of the executant on the other. It is not a 

 matter to be determined by rules ; personal equation, sometimes 

 idiosyncrasy, determines how the light shall be regulated. Many 

 finished micro-draughtsmen use a feeble light in the image and a 

 strong light on the hand and paper, and others equally successful 

 manipulate in the precisely reverse way. But upon the adjustment 

 of the respective sources of light to the personal comfort of the 

 draughtsman will depend his success. 



Care must be exercised in this work in the case of witical images. 

 These must not be sacrificed either by racking the condenser into or 

 out of focus, or by reducing its angle by a diaphragm. If the in- 

 tensity of the light has to be reduced, it must be done by the inter- 

 position of glass screens, and this is beautifully provided in Abbe's 

 camera. The illustration of how the various apparatus for the use 

 of the camera lucida should be disposed, given in plate III., may be 

 profitably studied. Both mirror and bull's-eye are turned aside, 

 and the hand and pencil are illuminated by the shaded lamp. 



The lamp illuminating the image is seen, with such a screen of 



