SUNDRY APPARATUS 



73 



of the theory of light, which is not within the scope of this 

 book. 



An eyepiece with a movable pointer or indicator (Fig. 83) is Demon- 

 , a useful aid to teaching. It consists of an eyepiece magnifying ly^'w. 

 X 10, which has a fine movable index 

 which can be made to point to any portion 

 of an object under consideration, or can 

 be turned out of the field when not re- 

 quired. It is invaluable for demonstra- 

 ting ceU-structm-e, crystals, etc. 



An eyepiece with a pair of cross hnes 

 (Fig. 84) is necessary for petrology where 

 angles are to be measured by means of a 

 rotating stage, and is useful for other 

 purposes. 



An eyeshade (Fig. 85) which clips Fia. 83.— No. 3263, Eye- Eyeshade. 

 on to the drawtube of the microscope piece with Indicator, 

 obscures the unemployed eye and saves 



much inconvenience and eye-strain with a monocular microscope. 

 It enables the observer to keep both eyes open without his 

 attention being diverted. 



An erecting eyepiece is a very low-power eyepiece which Breoting 

 does not invert the image. It drops into the tube *^°p'*™" 

 of the microscope in the ordinary way, and is 

 made for use with a 2/3-inoh object glass. It 

 gives a magnifying power from 10 to 40 diameters 

 by extending the drawtube. It gives a very large 

 field of view and an erect image, so that it at 

 once converts an ordinary microscope into a thor- 

 oughly efficient dissecting microscope ; and a slight 

 alteration in the length of tube gives great varia- 

 tion in magnifying power. 



To take photographs through the microscope which are Photo- 

 entirely satisfactory for most purposes is simple, and does """"^^p*"^ 

 not require much apparatus or special appliances. 



The microscope is first 

 arranged to give the best 

 visual image, the particulars 

 as regards illumination given 

 in the earlier part of the book 

 having been carefully followed. 

 The ordinary eyepiece is re- 

 placed by the 30-mm. focus 

 compensating eyepiece, and the 



photomicrographic camera is attached to the tube of the micro- 

 scope. The image must now be carefully re-focussed upon the 

 ground glass and the plate-holder inserted. The light is cut 

 ofE from the microscope by placing a card between the light and 



Fio. 84. — 

 No. 3264, 

 Cross Lines 

 of Eye- 

 piece. 



Fig. 85.— No. 3257, Eyeshade. 



