82 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



image of the useful portion of the object under examination ; and the 

 opening of the screen F a height equal to half the dimensions of the 

 image of the upper lens of the objective. 



A is the object examined ; C the eye-piece. 



Gilbertson's Microscope. — This old non-achromatic Microscope, 

 kindly lent for exhibition by Mr. H. E. Freeman, and since presented 

 to the Society, was made by H. Gilbertson, of London ; the date of its 

 manufacture, however, has not been determined. The instrument was 

 evidently designed for use in the field, for on reference to fig. 2, drawn 

 by Mr. Parsons, it will be seen that it is essentially a hand Microscope, 

 without either stand or stage, but in place of the latter there is a com- 

 pressorium, or live-box, fitted to the end of a tube which slides over the 

 body-tube of the Microscope ; this sliding tube constituting its sole 

 focussing adjustment. 



Fig. 2. 





u 



Jl 





I 



Fig. 3. 



A lower power is obtained by removing the front lens of the objec- 

 tive ; the body has marks upon it indicating the position of the sliding 

 tube when either the high or low power is employed. There is one 

 drawback in the design of this instrument, and that rather a serious 

 one, viz. the only motion that can be given to the object is in the 

 direction of the line of sight for focussing; all other movements, such 

 as lateral motion in the field, are impossible. If the object, therefore, is 

 not placed with tolerablo accuracy in the centre of the compressorium, 

 it will not be visible in the field of the Microscope. Diameter of field 

 with high power, ^ in. Diameter of field with low power, A in. 

 Working distances, y,. and 1 T ^ in. respectively. Length with live-box 

 closed, 4^ in. Greatest diameter, 1 in. 



Fig. 2 represents the arrangement with the tube drawn over the 

 instrument, indicated by the dotted lines ; fig. 3 the compressorium 

 detached from the Microscopo and the cap removed. 



