122 Dr. G. C. Wallich on the Markings of the Diatomacece 



white spot behind the eye; prothorax slightly corrugated in the 

 centre, a broad yellowish-white line on each side and a narrower 

 one beneath ; scutellum transverse, black ; elytra acuminate, 

 finely punctured, pale brown, with two longitudinal whitish 

 lines on each, united at the shoulder and apex ; antennae and 

 legs pitchy, the former nearly three times the length of the 

 body, the femora reddish at the base; body beneath with a 

 greyish pile. Length 7 lines. 



Cylindropomus should, T think, be placed in the Lamiidse, 

 near Olenocamptus, from which, indeed, it scarcely differs. A 

 series of this species shows a very considerable amount of varia- 

 tion in the width of the head : in some it is scarcely wider than 

 the prothorax, and from these it runs up to nearly twice the 

 width. 



XIII. On the Markings of the Diatomacece in common use as 

 Test-objects. By G. C. WALLICH, M.D. 



THE markings on the valves of certain species of Diatoms have 

 long been prized as tests of the defining arid amplifying powers 

 of microscopic lenses. Up to a very recent period they answered 

 every purpose ; but the rapid advances achieved of late by our 

 leading opticians in the construction of objectives furnish us 

 with many instances in which the formerly received interpreta- 

 tion of minute organic structure has turned out to be fallacious. 

 These tests have thus been somewhat unduly valued, and, as a 

 natural consequence, many indifferent objectives have been 

 thrust on the public, the efficiency of which depended, not on 

 accuracy of construction, but on the variable nature of the tests 

 they were subjected to. 



Accuracy of measurement is of the first importance in all 

 microscopic investigations, whether we desire to measure the 

 striae on a Diatom, the thickness of a cell-wall, a blood-disk, or 

 any of the numberless objects the microscope reveals to us ; and 

 until this perfect accuracy is ensured, it is almost needless to say 

 that a vast amount of time and patient labour will be expended 

 in vain. 



As an example in point, we need only take the value of Pleu- 

 rosiyma fasciola, a Diatom which was considered, until a very 

 late date, as one of the severest tests for a first-rate J or T '^ ob- 

 jective. It is true that P. fasciola, under certain restrictions, 

 may be made an admirable test ; but it is one adapted with equal 

 propriety to test either a | or a j^ objective. This is due to 

 the widely varying character of its lineation, a fact of which 

 the less scrupulous class of opticians were not slow to avail 



