March jo, 19 13] 



NATURE 



61 



upon any feature to which he wishes to direct 

 attention. 



The arrangement of the device is shown in the 

 subjoined figure : — 



Fig. 2. — Demonstrating eyepiece. 



1 and 11 are two prisms in contact and mounted 

 above the diaphragm between the field-lens and 

 the eye-lens of the eyepiece. The prism I has an 

 isosceles cross-section, and its angles are 35 , 35°, 

 and no respectively. The prism II is rect- 

 angular, and its angles are 35 , 55 , and 90 . 

 The prisms are placed with those faces in contact 

 which subtend the angles of 90 and no° in such 

 • a manner as to leave between them a very thin 

 film of air. This film is inclined at an angle of 

 30 to the axis of the eyepiece and partially 

 reflects the emerging pencil of rays ; about two- 

 thirds of the rays pass through the prisms, and 

 one-third is reflected. 



The image formed along the axis of the micro- 

 scope is accordingly brighter than that produced 

 by partial reflection. The centre line of the 

 reflected pencil is inclined at an angle of 70 to 

 the axis of the microscope. Ill is a prism the 

 lower surface of which reflects the pencil upwards 

 at a convenient angle for observation. In order 

 that the two observers may' not be in each other's 

 way, the branch tube is fitted with a system of 

 lenses which resembles a terrestrial eyepiece. 

 The image as seen in the side tube is reversed with 

 respect to that which appears in the axial eye- 

 piece ; but this scarcely affects the observer, since 

 the oblique attachment of the side eyepiece 

 changes the orientation of the field which is 

 focussed through the principal eyepiece, as the 

 image seen through it is brighter. The adjust- 

 ment for one eyepiece furnishes a clearly denned 

 image in the subsidiary eyepiece, provided the 

 eyes of both observers can accommodate in a 

 similar manner. The objective in conjunction 

 with the field-lens below the double prism of the 

 two eyepieces forms an image in the plane of 

 the diaphragm below the double prism. This 

 image and the pointer, being both in the plane of 

 the diaphragm, are seen simultaneously in both 

 eyepieces. The pointer can be moved backwards 

 and forwards and turns on a pivot so that its 

 extreme end can be set to any point in the field. 



The new eyepiece is well adapted for the 

 instantaneous photography of living bacteria and 

 no. 2264, VOL - 9 1 ] 



other moving organisms illuminated by means of 

 a dark-ground condenser; it enables one to 

 observe the object through the side eyepiece and 

 to defer the exposure until a favourable moment 

 presents itself. 



This eyepiece makes the instrument 

 to which it is attached into a binocular 

 microscope in a new sense. Its use will 

 certainly not be confined to labora- 

 tories ; it will equally be a delightful 

 acquisition to tyros discussing "pond- 

 life or other subjects in which amateurs 

 take an interest. The 6-diameter power 

 is to be preferred, and as the branch 

 tube is not counterpoised, if the eye- 

 piece tubes do not fit tight it is better 

 to use the microscope in a vertical 

 position. 



STANDARDS AND TESTS FOR SEWAGE 

 AND SEWAGE EFFLUENTS. 



THE eighth report of the Royal Commission 

 on Sewage Disposal deals with the im- 

 portant question of standards and tests for sewage 

 and sewage effluents discharging into rivers and 

 streams. In their fifth report the Commissioners 

 indicated the desirability of fixing a legal standard 

 for .sewage effluents, and suggested that such a 

 standard should be based on (i) suspended solids 

 and (ii) absorption of dissolved oxygen. Their 

 contention then was that the two tests should be 

 taken separately, and they suggested three parts 

 per 100,000 as the limit of suspended solids, and 

 that the effluent after removal of its suspended 

 solids should not absorb more than 05, 1, and 1*5 

 parts dissolved oxygen per 100,000 after one 

 day's, two days', and five days' incubation at 

 65 F. respectively. 



In their present report the Commissioners 

 recognise the difficulty of the separation of the 

 suspended solids, and finally recommend the fol- 

 lowing as the normal legal standard, viz. : 3 

 parts per 100,000 of suspended solids, and, in- 

 cluding its suspended solids, the effluent shall not 

 absorb more than 2 parts dissolved oxygen per 

 100,000 after five days' incubation at 65 F. 



The importance of this report lies in the fact 

 that not only is a definite legal standard recom- 

 mended, but that in the opinion of the Commis- 

 sioners this standard should be a variable one, 

 dependent on the conditions at the outfall, 

 i.e. condition of river or stream receiving the 

 effluent and relation of volume of sewage effluent 

 to river water. 



The Commissioners state that their experience 

 leads them to think that if the dilution while not 

 falling below 150 volumes does not exceed 300 

 volumes, the dissolved oxygen test may be 

 omitted and the standard for suspended solids 

 fixed at 6 parts per 100,000, and if the dilution 

 while not falling below 300 volumes does not 

 exceed 500, the standard for suspended solids 

 may be further relaxed to 15 parts per 100,000, 

 and with a dilution of more than 500 volumes all 



