These defects are known as striæ or veins, and their presence in glass intended for the better kind of optical work renders the glass useless. As will be seen below in the production of optical glass, special means are adopted for the purpose of rendering it as homogeneous as possible; in fact, the early history of optical glass manufacture is simply the history of attempts to overcome this very defect. The problem is, however, beset by chemical and physical difficulties of no mean order, and even in the best modern practice only a small proportion of each melting or crucible full of glass is entirely free from veins or striæ. In many cases these defects are very minute, and sometimes escape observation until the stage of the finished lens is reached. At that stage, however, their presence becomes painfully evident from the fact that they interfere seriously with the sharp definition of the images formed by the lens in question. It will be seen that in such a case time and money has been wasted by grinding and polishing what turns out to be a useless piece of glass. Methods are, therefore, used for examining the glass before it is worked, whereby the existence of the smallest striæ can scarcely escape detection. These methods depend upon the principle that a beam of parallel light passing through a plate of glass will meet with no disturbance so long as the glass is homogeneous, but if striæ are present, they will cause the light to deviate from parallelism wherever it falls upon them. Under such illumination, therefore, the striæ will appear as either dark or bright lines, when they can be readily detected. One form of apparatus used for this purpose is illustrated in Fig. 14. [Illustration: FIG. 14.--Diagram of striæ-testing apparatus. _L_, source of light; _S_, slit; _A_ and _B_, simple convex lenses; _G_, glass under test; _E_, eye of observer. The arrows indicate the paths of light-rays.]