第十章 初唐数学第一课
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薛朗看平阳公主没让秀美女子避开，也没多说，干脆的开口：“将军，学生失礼。龙腾小说网ltxs520.com”
说着，站起身，拿过图纸摊开，直接讲解：
“这是三弓复合床弩。三弓复合床弩，最大射程可达两千步。床弩的优点是射程远，力量大，土夯的城墙也能在一射之下摧毁，守城、攻城都可以用的利器；缺点就是操作需要的人多，不方便移动。”
说完，拿过另外一张图纸，继续解说：“神臂弩，射程可达二百四十步以上，单人操作，可射穿两层铁甲。听闻公主将军担当重任据守苇泽关，为大唐之门户，外拒胡人，内抗乱匪，救命之恩无以为报，学生心中不安，奈何身无长物，这两份图纸公主如能用得上，那真是再好没有。”
平阳公主也是领兵打战的人，一听就明白其中的好处，一直安然淡定的眼神，终于多了几分神采，目光灼灼的望着薛朗：“这图纸的画法也颇为新奇，郎君可否指点一番？”
“这叫三视图画法，就是从平视、俯视、左视观察一个物体所得的图形，公主请看……”
薛朗上前，摊开图纸，详细给平阳公主介绍这个图怎么看。
薛朗从小学画，对设计类的专业十分感兴趣，后来虽然没上大学，但自己也自学了不少东西。
平阳公主捏着图纸，纤白的手指轻轻摸了摸纸张：“这纸也与我大唐常用的不同，笔墨也有奇异之处，是否制造的工艺有区别？”
薛朗见过甘草给的红纸，只不知书写纸是否有区别：“请将军把常用的纸拿来看看。”
平阳公主从桌上抽出一张纸递给他。
薛朗是学画的。当年父母没出意外时，他跟老师学画的时候，第一节课就被老师带着认识各种纸张，外出写生的时候，经常带着薛朗去各种传统工艺制纸的地方参观。老师常说学画的怎么可以不了解纸。
薛朗上手摸了摸，迎着光看了一眼就看明白了：“公主常用的是麻纸，主料是麻类植物；学生这个纸是皮纸，制纸的主要材料是树皮。至于笔，学生画画用的是铅笔，写名刺的那个是钢笔。在我们那里，毛笔一类的称之为软笔，钢笔一类的叫做硬笔。追求书法艺术的才写毛笔字，常用的还是硬笔，书写方便、快速。”
钢笔薛朗有随身携带的钢笔，从兜里掏出来递过去并告知拧开笔套的方法。
平阳公主试着在纸上写了写，歪七扭八的，显然不习惯。不过，这钢笔的好处和方便之处倒是一眼就能看出来。
平阳公主颔首，放下图纸，拿起望远镜：“这又是何物？”
薛朗：“这叫高倍望远镜。”
“望远镜？这是用来登高望远的？不过，何谓高倍？”
薛朗又解释了一下什么叫高倍望远镜。还好他没买带夜视功能的，要是买个带夜视功能的，一穿越基本就废了，没电池啊！
“此物于行军上倒是颇有用处。”
平阳公主一眼就看出其中的好处。薛朗笑道：“确实有用，一路上因为有这个，避开了不少危险，不然，别说回归中原，就是活命都有问题。”
平阳公主点点头，也不急着去试用，而是问薛朗：“此物颇为奇妙，郎君可知是如何制成的？”
“望远镜的原理很简单，就是利用透镜的光线折射和小孔成像的原理。原理是不难，问题是材料！学生这个望远镜，看的时候，能大致测出远近、大小，其中最重要的就是镜头！就是这里！镜头的好坏，关系到观测到事物的清晰度、远近程度，测距需要的是计算。”
“计算？如何计算？”
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