102 DEPARTMENT OF THE NAYAL SERVICE 



ed while still fresh, then dipped in clean water, and laid in their wet state on an 

 object glass, upon which have been placed three small drops of white of egg and gly- 

 cerine (half and half), one little drop to each scale. Care must be taken that the 

 scale lies with its inner side against the glass. When the water has evaporated, the 

 scales will be found adhering firmly to the glass, and a permanent preparation is thus 

 cbtained. Notes as to length, weight, etc., may be made on the glass itself with water- 

 proof ink (India ink to which is added some water-glass). 



A third method of extreme simplicity is merely to take a single scale from each 

 fisli, and place it in a tube of water, to be mounted later. Tubes are kept for each 

 length-group, or for each length-group and either sex, and the scales are then placed 

 each in the tube assigned to its particular length and sex. Each scale in a given tube 

 thus represents a fish of a certain sex and length and may afterwards be mounted on 

 glass slides. By t3iis means, a large sample of herring specimens may be dealt with 

 in a very short time; it involves, however, the necessity of dispensing with data as to 

 weight, state of development of genital organs, etc.. besides lacking the indubitable 

 advantage conferred by having several scales from the same fish. The advantage of 

 the method lies in the fact that it enables the operator to procure a large qiiantity of 

 material for age determinations in a short space of time, and that it may be employed 

 even under the most unfavovirable conditions. Moreover, where no examination of 

 the specimens is made as to sex, the fish are entirely unharmed. 



Microscopical examination. — As we have seen in the description given in the 

 foregoing of the structure of herring scales, the impression of the winter rings is 

 produced by reflection and refraction of light in the outer surface of the ocale. In 

 accordance with this, it has been found that no colouring, or similar process, will serve 

 to render the rings more distinct than they naturally appear. The most that can be 

 done is to alter the conditions of reflection and refraction by embedding the scale in 

 a more or less highly refracting medium, experience having shown that this does 

 render the winter rings more easily discernible. For examination of herring scales, 

 a medium with not too great power of refraction has been found most useful, water 

 with a little glycerine, or alcohol (90 per cent) is good, the latter being preferable 

 when the scales have been mounted with the white of egg and glycerine, as aqueous 

 liquids tend to loosen the mounted scales. 



In observing scales through the microscope, a suitable attachment should be 

 used, preferably an objective so arranged that the power can be altered at will. Leitz' 

 objective la and Reichert's objective Ih are both very convenient, as with these the 

 power may be continuously changed within certain limits. 



In age determinations, it is best to remove the condenser from the microscope, 

 and leave the lighting to the plane mirror alone. It is rarely possible to obtain suit- 

 able lighting of tlie entire scale simultaneously, and the mirror must therefore be fre- 

 quently moved. The rings are best seen in slightly oblique light, when they show up 

 darker than the summer zones {vide plate I, fig. 1). 



In counting the winter rings, the operator should make it a rule to commence at 

 the margin of tke scale and work inwards towards the centre, i. e., first counting the 

 rings which are most difficult to distinguish. 



Where several scales have been preserved from each specimen, it is well to make 

 a preliminary glance at all those which are ready prepared, and choose the one in 

 which the rings appear most distinct. When the rings on this have been counted, 

 another scale may be taken as a check, to see if the same number and arrangement of 

 the rings is also fouiid there. 



Instead of counting the rings on the scales, it is nossible, by means of a drawing 

 mirror, to outline them on paper and count them afterwards. Such a method pre- 

 sents several advantages a^so as regards the actual determinations of age. and it is 

 a question whether the method should not be adopted in most cases. It is best to 

 use a drawing mirror with a series of smoked glasses for regulating the light. In 



