1052 



RECORD OF CURRENT RESEARCHES RELATING TO 



somewhat, or more carbolate added. The solution can be made in 

 about one hour. 



No fear need be had about boiling too long, as the longer this is 

 done the less liability will there be for the solution to deposit crystals 

 on the bottom of the bottle when cooled, which it will do if the tem- 

 perature is only kept up long enough to first dissolve the crystals. 

 Some made in October 1879 is still free from any deposit. Filter while 

 hot, and the microscopist will have a solution practically of fluid 

 croivn glass as clear and transparent as glycerine itself, having only 

 one objection, viz. when of 1 • 50 to 1 • 525 index, the consistency is 

 such that if used on a histological preparation just mounted and the 

 objective racked back to remove the slide, the cover, unless great care 

 is used, will be lifted enough to endanger a choice preparation. 



Mr. Bragdon is still experimenting with the view of finding a 

 medium a trifle more fluid so as to make the homogeneous-immersion 

 objectives " as nearly perfect as possible for every -day use." 



Iris Diaphragms. — To the " Working Microscope " of G. Wale * 

 an inexpensive and very simple and ingenious form of " iris " is 

 adapted, shown (separated) in Fig. 121. It consists of a piece of 

 very thin cylindrical tube A, about £ inch in length and | in. diameter, 



the whole circumference of which is cut 

 through with shears to nearly its whole 

 length at intervals of about ^ inch ; by 

 means of a screw-collar B, attached 

 below, this cut tube is forced into a 

 parabolic metal shell (contained within 

 C) whose apex is truncated to an aper- 

 ture of about I inch ; the pressure of 

 the screw causes the thin metal tongues 

 to turn and to overlap in a spiral which 

 gradually diminishes the aperture to the 

 size of a pin-hole. On unscrewing the 

 collar B, the spiral overlapping of the 

 tongues is released somewhat, and their 

 elasticity causes the aperture gradually 

 to expand. 



As adapted to the stage of the 

 " Working Microscope," the iris, when 

 unsci-ewed until its aperture is smallest, 

 is then almost in contact with the base 

 of the slide ; when at its largest expan- 

 sion it is about y'^ inch lower. The whole device is fitted into the 

 opening of the stage from beneath (so as to be flush with the upper 

 surface) with one turn of a very coarse screw on the edge of C — a 

 far more convenient plan than the " bayonet joint." 



Another form of " iris " (Fig. 122) has been manufactured in 

 America by Messrs. Sidle and Poalk (fitting to their " Acme " 

 stand).! I^ is similar in construction to the earlier forms Imown 



See ante, p. 1045. 



t See this Journal, ante, p. 522. 



